Albino BlacksheepYahoo! Messenger
(Formerly, Yahoo! Pager)
Every major Internet company wanted in on the Messenger trend, and Yahoo! being the biggest, jumped in with Yahoo Messenger!
I probably would have never installed it for actual use back then (just to try it out for curiosity's sake), but it became popular around Albino Blacksheep in 2003ish due to its webcam chat feature. The members would even embed the webcams up on the site, which took some technie-knowledge if it were any other software.
Now, with no real advantage or exclusive features, Yahoo! Messenger seems to be dying down amongst the competition, along with the required @yahoo.com email address needed to use it. I can't, however, speak for everyone. Certain regions and online communities have their messenger of choice. After ABS gave up on Yahoo!, another place may have given it a try. It's still very popular amongst other webcam communities.