While not confirmed, there's a purported Android-powered Google TV set-top box currently in development, which "is likely to have third-party developers work on it to make applications built for a television set." This will most likely give Apple TV a run for its money, unless they start offering apps as well.
In fact, the Apple TV is entirely closed right now, as only Apple is able to modify its software (without hacking it, of course). I suspect that will change, following this revelation.[via Techcrunch]