[Here's fring running on my HTC TyTN 2, I'm tweeting!]
My friend Jomz keyed me in on this free app called fring. It's a IM and VoIP application. I've downloaded the version for Windows Mobile and am running it on my TyTN2. It supports [as of this writing] Skype, Gtalk, MSN, Yahoo, AIM, twitter, and ICQ. Not bad :D
It can use cellular, wifi or even activesync to connect you to your friends... pretty neat.
Go to: http://www.fring.com to see if your mobile is supported :D it's free and that's fringing enough reason to take a look see!