Meetup.com is a good place to start. Also google phrases like "linux user's group" + cityname, "ruby user's group" + cityname, "java user's group" + cityname, "perl mongers" + cityname, etc., etc. Once you find a handful of interesting groups to start interacting with, you'll usually find that one group leads you to another (ex, groups cross-posting meeting announcements to one another's mailing lists, or standing up and making an announcement about an upcoming meeting, at the meeting of a related group, etc.)
Edit: plancast is another decent place to find out about upcoming events of various sorts.