I'd use Skype if my microphone didn't break. Yahoo is just ewww. Google Chat is great, but not used by any of my friends. AIM scares me a little. So MSN was the last alternative xD
prefer yahoo!, but a lot more seem to use MSN. I was reading a while back that YahoO! and MSN are working on a deal that accounts will work on either client. Don't know where that is at though. Use gain here, so it really doesn't matter for me yet...
We use googletalk here at the office. Just installed it at home too. My wife likes Yahoo to talk to her friends, and google to talk to me.