I recently came across this Instant Messenger client that supports:
* MSN
* AIM
* ICQ
* Y!
* GoogleTalk
* Gadu-Gadu

What I find to be most excellent about this client - is it runs great on Windows Mobile (5 and 6); has a really good GUI; and supports both voice in the IM and video (but only to other Palringo clients I assume).

I don't run this on my desktop (at least not yet) - but it is my cell phones exlusive client now - I highly recommend it for your Symbian/Java/or Windows based phone.

Palringo Vocal IM