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 Spend four months ignoring new technology, and you're out of date.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.