I've used MSN Messenger Live, Windows Messenger, AIM, Trillian, ICQ, XFire, Yahoo
MSN Messenger Live/Windows Messenger - You will get the least spyware and the most functionality out of these 2...I recommend going with the Live version.
AIM - This is what everyone in college used. This also pops up a webpage when you log in. This can be turned off in the options. This does have advertisements running at the top part of the window...but rarely gets in the way...I don't even notice it anymore.
Trillian - This is a client that allows you to bring all of your IM accounts (MSN, ICQ, AIM, etc..) into one account. Pretty cool but you seem to lose some functionallity of the other programs
ICQ - Old news...haven't used in years...don't think it is as popular anymore
XFire - I use this as a chat for online gaming. The application will automatically download new patches for your games and let you know when they are ready. It allows you to chat with buddies in-game and has a couple of other cool features for gamers.
Yahoo - Worst product ever! I urge everyone to not even touch this product....they install so much crap on your computer when you install this product it's pathetic.
There's my $.02
