E-buddy:
By using e-buddy you can connect with yahoo, msn, google talk, facebook etc and start chatting.
Direct download link: just go to get.ebuddy.com from your mobile browser.

Mig33:
Chat online in real-time with millions of other users! Check your buddy list to see who’s available or block users to protect your privacy. You can also set alerts to let you know when your favorite people are online. IM your friends using compatible instant messaging services like MSN, Yahoo!, AIM, GTALK on your phone.
Direct download link: just go to wap.mig33.com from your mobile browser.

Nimbuzz:
Nimbuzz Mobile lets you call, chat, message, and send files on the go, for free. It combines all your buddies from Skype, MSN, Yahoo, ICQ, AIM, Google Talk and more. One login, one contact list, all accounts.

Direct download link: just go to get.nimbuzz.com from your mobile browser.