Aol instant messenger, ever since the days of my 400mhz Compaq Deskpro with the slow as molasses AOL 56k.
It works, most of the people I know use it.
I also use Teamspeak to get ahold of people directly considering I'm an administrator for a large gaming community, I don't know if that's considered an instant messenger.
Trillian was nice, I enjoyed being able to run Y!, MSN and AIM and have them all in the same place, quite convenient, although when I used it it didn't have tabbed windows which drove me nuts.