Though I have used MSN, Skype in the past but now finally settled with Yahoo and sometimes the ubiquitous gtalk in gmail
I'm using yahoo IM, it seems that everybody I know has had it the longest. A new one gaining popularity is the myspace IM and I'm starting to use it as much, but not quite, as much as the yahoo. Glad to see you doing a poll to see which is the most popular. Thanks
Yahoo IM, FTW! I use it for yahoo and MSN contacts. I've used gTalk once and I wasn't very impressed, but it was better than sending messages through PMs.