I love idling in IRC channels, it's a classic pastime of mine, so I might as well add one more to the list.
As far as mass group chatting goes, at least as far as setting up a semi-permanent space is concerned, it seems like people just continue to use IRC. I haven't heard of any decent cross platform, always-on chat things that are commonly used by anyone, other than funky proprietary Java based stuff that TV networks and that sort of thing occasionally put up on their site, but they always seem evil and inefficient (and are, of course, tied to a browser window). Seems weird in this light but I think people still either use IRC or (for non long-term stuff) just use IM... or they don't chat at all, and use a web forum.

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* Or things like in-game chat have taken their place maybe? But that's not so good for general purpose chatting is it?