VLC player - plays just about any media file you throw at it.
Firefox - works just as nicely on Linux as it does on your Windows box.
Thunderbird - sister program to Firefox is a robust email organizer like Outlook
Openoffice.org - is basically a free version of Microsoft Office (which isn't preloaded on most PCs anyway) and will even use Microsoft Office files.
Gimp - is Photoshop, but free (and again isn't something that is preloaded with Windows PCs)
Gaim - is a multi-messenger that IMs with most instant messenger programs
These few programs really cover just about everything the average PC user really does with their PC. And not that many AVERAGE users use Photoshop but I figured I would throw that one out there anyway. And I only covered what normal people do. There are tons of apps for Linux which realy do just about anything you could want. On top of that, many distributions of Linux come with neat little upgrade consoles that list all the free plugins and applications that you can add with the click of a button, without having to scour the internet for them.
Perhaps the coolest feature of Linux is the portability. Many distros are lightweight, fast and able to be installed and booted from just about anything. I think I heard about a guy booting his PC with Linux from the onboard memory of his printer once!
My favorite distros are the Live CD versions. These guys boot right from the disk into RAM meaning you don't have to install it at all, nor do you even need a hard drive. Once my HDD but lost none of computers functionality because I had a Live CD disro laying around. I popped it in right after the HDD failed and was right back on the net surfing along as if nothing had happened. Using a Live CD, you can test out Linux without messing up your existing OS.
My favorite one right now is Linux Mint 7. Burn a copy and Try it out!