There's currently no Jailbreak for the third-generation Apple TV. But if you have a first or second generation Apple TV, it's worth it. In fact, second-generation Apple TVs are selling for more than double the price of a brand new third generation Apple TV because of this.
A jailbroken Apple TV will let you run other kinds of apps and streaming services that aren't normally allowed by Apple.
If you have a compatible Apple TV model, it's a relatively simple process to jailbreak it if you have a Mac. Lifehacker has a great tutorial to get you started.