I use
BeeJive. It's not free but if you're serious about IM, it's worth it. It even does push-email alerts when someone's trying to chat with me (since the iPhone doesn't support background apps).
It's been around on other phones forever. It's a well-established application, with well-established support and a very professional approach.
They also have a free, web-based version that only works in Safari on the iPhone itself:
http://iphone.beejive.com (I haven't used the web-based one, so I have no clue how it works).