1. The Gym. The key is to talk to someone while they're BETWEEN exercises. If you start hitting on them while they're running, or in the middle of a set, your chances go way down.
And, if you're a guy, don't go up to a woman and comment on how toned one of her body parts is. It won't sound flattering. You'll just come off like a creeper.
2. Concerts. People at concerts are already in a friendly mood, and it's easy to start a conversation: All you have to do is talk about the band. It also helps that everyone's a little buzzed and relaxed. It's like a bar without the pressure to meet someone.
3. Volunteering. The fact that you're volunteering in the first place shows that you're passionate about something, which is pretty much universally attractive. And since THEY'RE passionate about the same thing, there's a chance you'll have some chemistry.
4. Any Kind of Team Event. As in, go join a kickball league or and Ultimate Frisbee team. It doesn't matter waht it is, just as long as it involves a large group of people.