Xfire and Windows Live Messenger are what I use. Xfires useful for finding what games your friends are in, and its got that neat little hours counter and stats image
...Windows Live Messenger is what I use for all my non-gaming friends (i.e off chat websites...and other ppl).
Use what you want, but I don't use trillian or any other...cause theres no need to have a program that you very rarely use most of its features for (i.e Trillian has Yahoo Messenger and AIM...both of which I never use).
Xfire can be laggy when you have dialup or 256k DSL...but it very rarely lags me up at all. Normally WMP lags everything up badly....