I used to be a hardcore Trillian fan, but I've switched to Pigin. The free version of Trillian doesn't support Google Talk or My Space Messenger, or at least didn't as of when I switched to Pigin. My friends switch services all the time and Pigin seems to keep me in touch with all of them.

I have a little program called I downloaded years ago called Citrus that lets you use your computer as an alarm clock. You can set multiple alarms and use MP3's for the alarm. It's a useful way to remind yourself to get up from the computer and do things.

I use the IE Tab plug-in for Firefox and just added the Chrome version, mostly because Firefox by itself seems to have problems playing nice with video on Slate. Foxy Tunes lets you control your mediaplayer from inside your browser.

C-Cleaner will clean up errant junk on your computer and make it run faster.

GIMP and Open Office give you some nice open-source productivity tools.