Oh, might as well toss these here too, some handy apps I install on pretty much every Mac I use now:

Adium (IM application)
http://adiumx.com

Quicksilver (Quick program launcher. Use it initially by doing Ctrl-Space, and typing an application name to launch. Does much more, search for Quicksilver tips video when ready to learn more)
http://quicksilver.blacktree.com/

Unarchiver (Unzips rar, 7z and other less commonly found archives, and stuffit. Works like the built in OS X unzipper):
http://wakaba.c3.cx/s/apps/unarchiver.html

Perian (Quicktime codec to play most videos, including divx/xvid and so on):
http://perian.org/

Flip4Mac, (plays WMVs in Quicktime)
http://www.microsoft.com/windows/windowsmedia/player/wmcomponents.mspx

Burn (Handy disc burning app for when disk utility/finder don't do what you need):
http://burn-osx.sourceforge.net/

Handbrake: (DVD to MP4 video ripper)
http://handbrake.m0k.org/