Unicycle Hockey.
Does exactly what it says on the tin.
Quite possibly one of the most fun and tiring (Yes! Tiring) sports I have ever taken part in. It's played, similar to normal field hockey, by two teams, with a goal at each end of a pitch. Also, more like Ice Hockey, you can take the ball (or puck) behind the goal.

Vital Equipment:

1. Unicycle, obviously.
2. Ice hockey stick.
3. Ball, puck, your preference really, but balls are best.
4. A "Goal" of some nature, a bag will suffice if you're only playing in the local park. Just has to be a target that won't be too ridiculously easy or hard to hit.
5. Some people! The more the merrier.
Unicycle Hockey is a great social game. The number of people who you will find come up to you and talk to you, just because you're riding a unicycle is incredible!

Another great thing about unicycle hockey is the fact that it really helps you to learn to unicycle, but it does require a certain level of ability before playing. Once you can handle riding in a straight line, and can handle holding a rather cumbersome ice hockey stick, which can be used to help you stay upright! Very handy if you're not very good (like me). If you play regularly, you will notice a huge improvement in your unicycling ability, as you have to be able to turn and change direction very quickly, and as you play more and more, you will learn how to do this.

The only problems with unicycle hockey are that it is a little expensive to get started, but if you can find a local circus skills club, or something like that, you are likely to find unicycles and unicyclists. Then all you need are some sticks. My advice would be to get the cheapest sticks you can get, because as you tend to lean on them a lot, the heads do get worn down a lot.
I would also highly recommend regular unicycle maintenance.

Believe it or not, there is even a kind of international league for this sport, although it's not particularly large.