JiveTalk is an instant messaging client for the BlackBerry that is head and shoulders above the competition. It provides data-based (as opposed to SMS based) instant message access that is fully functional. I’ve been using it for about a month now in closed testing, and it’s awesome. It provides access to AIM, ICQ, Yahoo, MSN, Jabber, and Google Talk. It’s the Adium of BlackBerry IM clients.
You can download it over the air from www.beejive.com.