User groups are great. In the IT business, learning new things about your field isn't just a nice idea; it's mandatory. Spend four months ignoring new technology, and you're out of date. Go four years without learning anything new, and you're a historian.
You need three things to quickly and effectively learn a new technology: access to the system, quality documentation, and someone to call on when you get stuck. User groups are a great source for that last item. The more people you know, the more people you can call on who may have had an experience you haven't--people who can give you that crucial clue to pull yourself out of the muck. And you can keep your eye out for a chance to return the favor.
This listing is incomplete, but my ambition is to make it all-inclusive. So far I've listed some groups I know are out there but I have no information about. If you know of a group that should be included here, please let me know.