mIRC, duh
I've always used DeadAIM, you need to use an outdated version of AIM for it to work right, but I hate the new AIM anyway
It never hurts to have a few installed.. I have IE7, Opera (main browser), Firefox, and Safari.
Personally I use BitTornado.
FCEUltra, ZSNES, Gens, VisualBoyAdvance, No$GBA, Project 64, etc.
For music.. Winamp, unless you have an iPod, for which you'll probably want iTunes.
For video.. Media Player Classic w/ QT and Real Alternative (if needed). Always get the latest ffdshow for your codec needs.