Google now offers chat from within gmail itself. No need to download chat/IM programs and instal them on your computer.
All you need do is log into your gmail account and see who's online in the new contact list that appears on the bottom left of your browser. Then simply "mouse-over" (let your mouse pointer hover on) the name of any contact and select if you want to mail them or chat with them. It's freakishly simple.
This has several advantages over other chat/IM clients. For one, as I mentioned, there's no time spent in downloading and installing software. This also means you can chat from any computer. All you need is an internet connection and a browser that's compatible with gmail.