I started with RedHat 4.1 (it must have been in 1998), which was frankly difficult to configure (GNU / Linux was for me a completely unknown at the time). I went through several Linux distributions (Mandrake, Suse). Today, I use Ubuntu (see below). I can be assertive, the choice for your future GNU / Linux, depending on your activity, what you do with your PC (for me …).
I reviewed numerous distributions, often by following their evolution and on different machines (486, AMD 100, Pentium 200 MMX, PII 400, PIII 500, PIV 2 GB, 2.6 GB PIV; AMD64 3400 +; [ ...] notebooks and desktops). I often use Qemu to test these Linux distributions without risk using a virtual machine.

a good site to select and download (ADSL desirable) the Linux distribution of your choice
You can order a CD for Linux distribution to http://ikarios.com (I have no connection with this company)
Free Linux distribution (for download, print or CD drives)
The notes are comprehensive with logos (my appreciation of the Linux distribution)
Probably the best Ubuntu distribution now.
This Linux distribution is running from a CD, is based on Debian and can be easily installed using the dedicated software. I prefer Ubuntu but now I recognize that from the “wonderful world of Windows” I had a little trouble at first with the GNOME interface. Now I wonder how I managed to do without it for so long!
Ubuntu is very informative. The hardware recognition is almost perfect. I use without problems in my Wacom tablet that I thought leaving the closet forever. The prints are finally free frame and I enjoy 3D.
The bundled software small but essential and especially simple enough for use in Live-CD that does not repel. If you put nothing prevents you to complete the list.
My current workstation: UBUNTU
I use virtual desktops with six gDesklets for applets (bottom menu so MACOSX, clock, weather and data system right) Conky (bottom left) showing the I / O network. Windows software were installed with Wine (icons on the bottom right).
Knoppix a classic
It runs from a CD, and is based on Debian, with an installation even easier. Knoppix is far more interesting than DemoLinux and faster. This time, there are almost 2 GB on the CD. Recognition devices at boot is good – I have too many problems (printers, storage, networking, USB2, sound card, …). Knoppix still demand a little more DIY than some other configurations for …
It is also possible to install on the hard disk – see setup with screen shots, and also see the tips. While the installer is a little too basic, it is nevertheless perfect for the beginner (because not excessive).
There may be some problems with devices too recent, but I think it’s rare (an update of the system – kernel – solves this problem). In practice, think “Debian”, when one is accustomed to a Mandrake or Redhat – records management is a little different. But totally effective!
I installed it without problem on a Pentium 166 with 64 MB Ram, and on another machine with only 32MB (running very slowly). I appreciate it a lot, especially since the CD is updated regularly, whenever necessary (several times a month).
Finally, it is possible to make its own version of knoppix. Go to the “documentation” of the site www.knoppix-fr.org
I stopped to use it because KDE crashed too often. Maybe it will arrange with future versions.
You can also use a great script that helps you step by step to creating your own CD: http://thefredsite.free.fr/linux.php?id=93
Here is a screenshot of my old desktop Knoppix 3.2 (click to enlarge):
In the picture are visible: KDE 3 with 6 virtual desktops; KuickShow, Cycas and radiosity rendering, Gkrellm with Xmms control and clock moon.
Derived from Knoppix
Everyone can make his “Knoppix”. They are found as well in different areas (see knoppix-fr) such as education, astronomy, etc.. …
Examples:
Kaella - entirely in French, it follows the main developments of Knoppix.

Kanotix – exists for PC 32 and 64 Bits – really fast, this version is easy to franciser and Kanotix team has made many scripts to help you install and configure your hardware.
64-bit version: ftp://ftp.fbihome.de/linux/kanotix/KANOTIX-64-current/
(720 MB) also applies to Pentium64)
Damnsmalllinux, made to run on a CD format “credit card” (50 Mb)

Also note, Freeduc, linux distribution dedicated to education (early childhood to high school), made entirely of free software, which also contains a manual and those of linux software.
Oralux, linux distribution for the visually and hearing impaired.
For more information: http://knoppix-fr.org/site/derives
Debian GNU / Linux a classic
Very stable, probably the best linux distribution, but difficult to access for the novice during the installation is done in text mode. It is useful if you want a good server, or if you want a reliable job. But beware, your devices will not always be recognized and will not be possible (without encountering major complications) to install all the software you want.
In short, Debian is an excellent choice for those who have a lot of patience and … “Bottle” …

Mandriva another good choice
Mandriva is the linux distribution to many Fcountry who brought the community Linux (Mandriva was born from the purchase of Connective Mandrake). It comes in several versions (public, corporate server). You can download it or buy it (the paid version is more important with many CD or DVD and includes manuals).
I tried Mandriva 2005 64bit (v10.2 beta). It is very satisfactory, especially by its excellent performance in the recognition of equipment (including USB2 devices, engraver and graphic tablet, generic Nvidia card).
Mandriva is now a rapid and stable. It is excellent for beginners (easy installation – a model of its kind).
Derived from Mandrake
There are linux distributions based on Mandrake, conducted by independent or Mandrakesoft (editor Mandrake).
MandrakeMove is published by Mandrakesoft. This is a perfect product to discover Mandrake without risk, or work with, like knoppix (MandrakeMove but not install). You can download it for free (flanged version, without the possibility of backup) or buy a USB key (multiple sizes available).
Hardware detection is correct, and the configuration tool is generally excellent. Given the configuration options, I urge you to buy it to save your changes. But some changes made by Mandrakesoft interface, make heavy their use for the beginner (especially the Cups printer driver). The present software is perfect: OpenOffice in French dictionaries are pre-installed, Totem (Xine interface) as a media player, etc.. …
The menus are clear and each use (recreational, office, internet, …) is well defined – there is no software too (many browsers and word processing, for example, as is the case with knoppix) not likely to disrupt the novice.
Linux distributions paid (for download or order CD)
Suse – another good choice
Suse is only available to buy, 90 euros for the professional version and 50 Euros for the personal version (which I do not advise you; utilities installation and configuration are lighter). But once bought, you are free to duplicate it on as many machines you want. However, there is a CD LiveEval (available in journals or download) type knoppix, but without the possibility of installation and indolent. Finally, it is possible to install from their FTP server (broadband connection required). Suse to the advantage of being almost as fast as Debian stable, being accessible to all and provided a very good recognition devices. The control center of this distribution is YAST2, often more powerful than drakconf of Mandrake.
Manuals in English are relevant and quickly proved indispensable.
Linux Distributions of basic (classic)
RedHat – now destined for the corporate world. It is replaced by the general public Fedora open-source project. The original was based on Mandrake RedHat.
Slackware – one of the oldest linux distribution, and also one of the most easily accessible to the novice. I have not tested.
Debian – see above …
Suse – see above …
In short, know also that most of the time, regardless of the linux distribution you choose, it will be provided with several graphical environments that do not change their method of use, a distribution to another.
Thus the main (Gnome, KDE) and will find it was little adapted to the linux distribution. This will allow you to change it quite easily. Finally, for those who come from Windows, I recommend using KDE, with the recommendation not to forget that Linux in general can not click once on the icon to launch the application. But it is now possible to set KDE and Gnome, to adopt the method of work of another system (Windows, Mac) following the use of the mouse and keyboard.
The update (free) from your linux distribution:
In general, when we set up a linux distribution, it is often sufficient to update via internet, for fixes to bugs discovered, and the latest versions of applications and drivers. But it is better to have a fast connection (ADSL), as this little be long and of course it should be done for each of your machines (there must update the network on some linux distributions). Therefore, I prefer to get the latest version on CD that I re-installed. It also saves me some nasty bugs, such as where the software update prevents choose what you want and takes ages to recover the files on the Internet. Or even, he will need an update on the operation and plant …
Linux distributions paid:
There are many linux distributions paid, their quality seems close to Susa. You can also buy free linux distributions which are then provided with more CDs than those found in the press or for download. Sometimes you will have the benefit of documentation (Suse) or poor (Mandrake), but with the whole phone support or email. You will also sometimes privileged access to a database on the Internet, regularly updated (Mandrake).
I found other linux distributions that seem correct but they are paid and it is not always possible to download a CD-based (free). At best, if it were still the case, it is obsolete!