Most new(old ones too) data capable phones have a built in IM client. In SE java phones, it is there in Messging menu with the name My Friends. Most of the people don't know how to use it. Although Java apps like Nimbuzz, Mxit, EQO, e-buddy and Mig33 are better performers at speed but for the occasional user, nothing beats My Friends. You just need to go to yamigo.com and register there. You'll get all setup instructions there itself. If you're stuck midway then do ask me here.
I'm busy now a days so blogging is getting little attention. Hope, I give much of myself to this blog after semester exams end. Bye bye:)