CCleaner (freeware): I love this app. I've never used a registry cleaner that does this good a job, with minimal risk. And it's fast.
http://www.ccleaner.com/download
The GIMP (Open Source): Awesome free photo editing program. I use it all the time. Also available on Linux and Mac.
http://www.gimp.org
CPU-z (Freeware): Great app for monitoring your CPU frequency, especially useful when overclocking.
http://www.cpuid.com/cpuz.php
CrystalXP (Freeware): Nice desktop shell replacement application. When you're bored with the Windows look, or fed up because you can't control exactly how you want it to look and function, this is nice to have. Just be sure to read and follow the installation/unistallation directions and you'll be golden.
http://www.crystalxp.net/bricopack/en.htm
Those are just a few of the apps I can't live without. There are plenty more, and I'm always trying out some newer ones as well. What apps do you like?

