So your friends use Yahoo Messenger, MSN Messenger, GoogleTalk, AIM, etc., and you would like to be able to IM all of them but don't want to install and run multiple IM programs. Then check out Pidgin.
I used Trillian for years and loved it, but it got a bit buggy at times, and I hated having to pay for an upgraded version just to use Google Talk. Pidgin works with so many more IM services.
For now, you still have to use other IM software for video and voice chat, but if you mostly do text messaging, then Pidgin is the way to go most of the time.
See more of my favorite software / programs (many of them free) at:
http://www.kristensguide.com/Computers/favorite_software.asp