The top five Linux distributions, or distros, are Ubuntu, OpenSuse, Fedora, PCLinuxOS and MEPIS. These rankings are courtesy of Distrowatch. You can go there and see which distros round out the top ten, hundred, and so on. There are literally hundreds of distros to choose from.

Now I'd like to get into the nitty-gritty of choosing and testing a Linux distro that is right for you. I'll try to stay as impartial as possible in this, though I may inject a personal fave or two into the mix. Please don't hold that (too hard) against me.

Two things to consider when choosing a Linux distro are your computer's RAM and Hard Drive Disk size. A third consideration, if you want all the eye-candy, is your video card. If you have more RAM (at least one gigabyte) then you can choose just about any distro out there and run it comfortably. Ditto for the HDD size; anything twenty gigabytes or larger should be sufficient.

The RAM and HDD size (and sometimes video card) considerations are two-fold; which window manager to choose, and whether you want to enable the considerable eye-candy now available to Linux users.
Yep, not only do you have to choose a distro, but you have to choose a window manager as well. Perhaps I should rephrase that; your choice of distro is heavily contingent on the window managers normally associated with that distro.

What is a window manager? Well, in Vista, it's Aero; in Mac OS X, it's Aqua. There are several RAM-hungry window managers available for Linux, most notably being:
KDE, GNOME, and Compiz/Beryl. KDE and GNOME are standard with most of the top distros, while Compiz/Beryl are window managers you can enable if you have sufficient RAM and video card capabilities. If you haven't seen the latter, then you're in for quite a treat. The lighter weight window managers (for those with less RAM/smaller HDD size) are Xfce, icewm, and Window Maker, among others. Please note that this is not a comprehensive list of all window managers, just a representative sample.

So, after taking stock of your computer's RAM and HDD size, you head over to distrowatch and choose a distro. Each of the distros listed has it's own page on distrowatch, with a listing of forums, download sites, galleries, reviews, mission statements, and so on. Be sure to check out some of the gallery shots before downloading your distro, as the visual appeal of your future OS will make a big difference in your computing experience.

OK, now you've browsed the galleries and picked out a distro. If it's a lightweight distro, such as Damn Small Linux or Puppy Linux (which, btw, can be booted from a USB thumb drive if so desired), then you can safely just download it from the download site or one of its mirrors. If you've chosen one of the bulkier distros (notably OpenSuse) then you may want to consider using BitTorrent,
as it's a four gigabyte download. Wouldn't want that baby timing out at the 3.9 gigabyte mark.

After downloading the ISO file, you're going to need to burn the disc image to a CD or DVD. In Windows XP SP2, Windows 2003, and Windows Vista RC1, you can use ISORecorder, (which is freeware) Nero (commercial) or MagicISO, among other choices. In Mac OS X, you can use Roxio's Toast software (commercial), or OS X's own built-in Disk Utility (found in the Utilities folder inside the Applications folder). In Linux, well, if you're using Linux, then you shouldn't have even read this far. You're probably compiling you're kernel from scratch while watching a keynote from Richard Stallman or the man himself, Linus Torvalds. Just kidding. In Linux you can use k3b, gnomebaker, or the built-in CD/DVD capability of the system itself (at least I can in Ubuntu and OpenSuse, ymmv).

You've now successfully burned the ISO file to disc. Open your CD/DVD drive bay door and insert the disc. Reboot your computer. As you reboot hold down the 'c' key on the keyboard and it will boot from the CD drive. Most of the top distros have a live CD feature which allows you to 'try before you buy'. You can browse around, making sure that the distro can see your internet connection and (hopefully) connect, set the correct screen resolution, and so on. I recommend checking if the distro has this feature before popping it in your drive bay, and of course backing up any important data before you do so as well.

If you choose to install, you have a couple of options; you can wipe the hard drive and install just the distro of choice, or you can resize your Windows install and have a dual-bootable computer. That means when you turn on your computer you can choose whether to go into Windows or Linux. If you're a Mac user, you can use many of the open source apps available to Linux users, as Mac OS X is built on the Unix-like FreeBSD system. Apps like Open Office, as well as a slew of others. If you're curious as to what's available to Mac OS X users you can go here and look around. Quite a large selection, in fact. I use Open Office on my Powerbook G4 and it's way better (and cheaper) than the Office suite from Microsoft.

You can also try the virtual environments of VMware and Parallels; VMware basic is free, Parallels is not, though you can do a 30-day trial of it if so desired. Another option for Mac OS X users is Boot Camp which allows you to triple boot (sacre bleu!) Linux, Windows, and Mac OS X. This is not virtualization, however, but a full install of the various systems. If you're not clear on the concept of virtualization, here is the Wiki.

A final consideration when choosing a distro is the community support. Ubuntu has by far the best forums for this, as well as the greatest worldwide net of mirrors for updating system software/adding apps. OpenSuse is also quite outstanding. This is kind of important, because if/when you run into a problem and want it resolved quickly, the forums are the best place to go. Especially when it comes to wireless issues.

Changing computing experiences can be a scary thing; for an overview of Linux/Windows meta you can have a look here (h/t douglas9). This is not a knock against either Microsoft or Apple. It just seems that the further their OSes have developed, the more locked in users are to a particular experience. The fun in Linux is the absolutely staggering amount of choice available. It reminds me a great deal of using Mac OS 7; yeah, it was horribly unstable, but you could customize to your hearts content. Small shareware firms were always bringing out cute little apps that added to the user experience. Linux is like that, only on a magnitude of order greater.

If you like a more polished experience, you can't go wrong with OpenSuse; if you are really into managing things down to the nth detail, then something like Gentoo would be your speed (be prepared for some wrinkles and/or hair loss, though). One of my personal faves is Linux Mint, which you can find here. Others swear by Sabayon and Zenwalk; alternatively, you might want to go with a professional, support version of Linux, such as Xandros, Red Hat Enterprise Linux, Yellow Dog Linux (made for Macs), as well as several others. Most of these provide free solutions in addition to their supported ones. If you're a true heretic (just kidding!), then you can try Solaris, FreeBSD, or OpenBSD. I would not recommend these for beginners.

If you have any other computer-related questions, don't hesitate to post them here; I'll do my best to answer them.