Been using Trillian a looong time love it no ads or other BS like you get with every other IM program now. Only issue I have with it is file transfers don't work sometimes.
I will add to the list thou.
Must have extension for fire fox = "Foxy Tunes"
Let's you control your favorite MP3 player from inside firefox volume control and some other nice features.
Another common one is Spybot search and destroy with "Tea Timer" has saved my ass more then a few times from programs install BS I don't need in the registry and also shady programs I've gotten sometimes after downloading torrents. :?
1994 Mustang Cobra Black
1995 Mustang Cobra Rio Red *RIP*
1999 Ford Ranger