I mostly use AIM. I don't use the AIM client, though; I use Adium. It's a multi protocol client that handles all the big names in IMing (like Yahoo, AIM, MSN) along with some you've never heard of (like Zephyr, QQ). It's like Trillian, but 19283 times better due to the customization options and lack of advertising and whatnot.
I got trillian running most of the time, and the messengers I have activated on it are MSN, ICQ and AIM. Yahoo acts up on my trillian sometimes so I have that open on it's own.