One more vote for Craigslist. I've found both good, paying, long-term gigs there and enjoyable little-to-not paying projects. Just be honest about what you are looking for in your ads and responses and ask straight shooting questions of the groups you answer ads for.
Even though Craigslist has actually been more reliable, my favorite way to find a group is word of mouth. I'd encourage you to put the word out, at any social or work environment that you are in. Church, work, clubs, etc, even (perhaps especially) groups that have nothign to do with music. You'd be surprised who knows someone who knows someone who plays and is looking for musicians.