Gaim -> Pidgin
Apparently, Gaim has made a settlement with AOL and had to change its name to Pidgin. The user interface looks pretty sweet from the screenshots. As far as I know, I think the program is still open source and now 2.0.0.
For those unfamiliar with Gaim (now Pidgin), it is a multi-platform Instant Messenger client just like Trillian. The main difference is that it gets rid of all the bloated bells and whistles and provides only the main necessities. People can still transfer files to each other and do just about all the things one would normally do with a typical messenger. Just think of Pidgin as the leaner and more efficient version of Trillian. It supports Windows and Linux platforms. Mac users might have to look deeper into that.
As always, people can mod the program with their own skins, buddy icons, and smiley themes.