What a sad mess the IM world is. Many users are locked in (in the sense that cross-network requires multiple logins / hacks)both because of their vendors, as well as their platform...
I have to cross-platform chat (mostly to Mac users), and I'd like the option of voice (I can only dream of video)as well as IM. That decimates the options out there. Most of them have reasonable windows versions, but poor hacked Mac ones. I also really dislike all the proprietary protocols.
For that reason Jabber is the bright hope for IM ( http://www.imfederation.com/
), none of the ugly proprietariness of MS/Yahoo/AOL. Anyone on a federated server can talk to each other. Both iChat and GoogleTalk are Jabber clients. GMail allows jabber chatting through a web browser which is neat.
One cross-platform free IM that has a very rich feature set (whiteboards, file transfer, voice, IM and lots more)and is wonderfully programmed (and actively developed) on all supported platforms is Bitwise:http://www.bitwiseim.com/
They use a proprietary network sadly, though they are at least clear that it is to do with security and privacy, rather than the market-domination strategies of the others. The forums seem friendly and helpful.
Though the hope for propietary free IM / voice (unless Google pulls its finger out and releases a cross-platform client) rests with the Jabber based Gizmo:http://www.gizmoproj....com/learn-more.html
I haven't tried it but it does voice / IM.
Of course, Skype does IM as well as voice (and file transfer)http://www.skype.com/
For IRC, I use Opera's built-in client - It uses Opera's rendering engine so I can style my IRC window using CSS. It does all I need.