IM:
YahooIM is still up and functioning, if I recall... But it still has issues, a lot of issues, hasn't had updates in a very long while and not many people use it anymore, as far as I am aware.
Skype, well, I'm sure you've used it
Tinychat has a mobile app, but I believe you have to pay for it, and it is pretty buggy. It also demands that everyone has to log-in themselves. It's basically an open chatroom, not an IM service.
Steam is pretty decent. The mobile app at least lets you message people on the go, but it does not enable group chats and voice chats here. It uses the chat bubbles for messages much as skype does on mobile. It's pretty bad lately on losing messages in cyberspace, though. It will also have random periods of having "steam hiccups" where the servers will go down and disconnect you, not allowing you to talk (computer and mobile) But on computer, at least, it doesn't have the horribly clunky skype interface and is pretty clear, great on screen use. You can set up groups for chatrooms and they also have forums pretty much you can make too, so even more utility there. Uhm, though, while it does have mic support, it's not the best in the world, last I used it.