RIM has created an Internet Messaging portal page for BlackBerry users. It provides a one-stop 'shop' of sorts that allows you to check out the latest IM software available for the Berry... and you can download each app too.
Currently offered are: AIM, Windows Live Messenger, ICQ, Yahoo! Instant Messenger and Google Talk.
At any rate, you may want to check this out on your browser, so punch this location into your Berry: http://www.blackberry.com/instantmessaging/