I don't use one but here's my buddy's company http://24im.com/features/livechat

to include in a website, jsjac is pretty nice. (http://blog.jwchat.org/jsjac/) it can be used to replace nearly all of your messaging functionality -- short messages, group messages, and live chat. it's reasonably easy to configure and install, and requires a jabber server running somewhere.

I'd imagine Campfire is popular.

irc