I'll start off by saying that I don't do much online chatting. It's something that I've just never gotten into. So I'm hardly an expert on this topic, but I'll offer what little I know and the research I've done.
Gaim is probably the most popular chat client for Linux; it is also available for other Unix-like operating systems, Mac, and MS Windows. Gaim is compatible with AIM, MSN Messenger, ICQ, Yahoo, IRC, Jabber, and several other protocols. This compatability means you'll be able to use your established account and continue to chat with your existing friends using an interface that is very similar to the chat client you are used to. The few times I have used it, I've found it to be feature-rich and easy to use.
aMSN is a MSN Messenger clone for Linux. I have spent a little time using this a few years ago and it was great. Eventualy I switched to Gaim because I acquired contacts that used other protocols and I wanted to use a single client to manage them all.
Some popular IRC clients include irssi and BitchX (also note that Gaim mentioned above is also an IRC client). I don't use IRC very much, but I check out some freenode chanels sometimes and have found the Firefox extension, Chatzilla to be more than adequate for my needs. I suspect that it has fewer features than many of the stand-alone clients, but for simple chatting, It has worked well for me.