I'm unsure of its maintainership status, but I usually point people who ask me this question to TrophyIM. It is a small and themeable chat client written on top of Strophe.js.
If you want group chat, you can try Speeqe (see it in action here), which is also based on Strophe.js.
Most people have very specific needs in the web domain, and I think this contributes to the lack of generally useful web-based IM clients. You can easily write one yourself, and there are several examples of such systems in my book, Professional XMPP Programming with JavaScript and jQuery.
Disclaimer: I wrote Strophe.js and the book so I am obviously biased.
