Add another vote for gaim over trillian. Trillian used to always crash, so gaim was better in that area. Plus, it just blends in better with the desktop and it's open source, which I support.
Tag & Rename - for organizing big mp3 libraries this is a must.
Somebody else mentioned keypass, I'll go more in depth into it (and programs like it). If you have any need to keep anything password protected, it's a good idea to use this. You can have it generate random passwords and store them so that when you need them, all you need to do is copy/paste. I've done this for every site and don't even know the password for some sites. Just be sure to back it up.
That reminds me: Portable Apps. If you have a flash drive and tend to work between computers you know how annoying it can be to have two different setups for firefox, etc. You don't need it anymore.
Portable Apps.com Lets you download portable versions of programs like Firefox, AbiWord, Gaim, 7-Zip, VLC, and others. They've been shrunk and don't need to be installed, so you can run them straight from your jump drive. So your firefox will always have the same history, bookmarks, and searches no matter what computer you're on.