Foxit Reader - a great replacement for the Adobe Reader program (free for personal use)
Notepad++ - Tired of trying to do anything with Windows Notepad? Download this badboy for a bunch of features including line numbering, super-powered find/replace (including regular expressions!), macros recording, tabbed windowing, syntax highlighting, and a slew of others.
PeaZip - PeaZip is an open source zip utility which supports compressing to and decompressing from a wide range of archive types.

VLC Media Player -: an open source video player which will play just about any video format out there
MediaMonkey - Not open source, but the best Windows music player I've found (better than iTunes and even Winamp, IMNSHO). Note: this software is 'shareware', meaning that you can purchase a 'full' version for money. The full version is what I'm actually running right now (got a free license through TrialPlay), but the free version is excellent too.

Launchy - Launch any program (or file) with a few keystrokes.
PortableApps - Portable apps is a collection of open source software that has been tweaked to run from a usb drive. Thus you can take all your favorite (windows only!) apps with you wherever you go.

+++ Better replacements for more popular software

TV and Movies online

Cool Videos

Dr. Horrible's Sing-along Blog - A Super Villain Musical written by Joss Whedon. Need I say more?
Things, but Very Slowly - cool, and sciency!