Apple recently added Hulu Plus to the AppleTV 2 (2010 and 2012 models only) so I wouldn't be surprised if they add more over time. There is even a rumor that the version of iOS 6 for the AppleTV gives developers access to the hardware so that they can produce apps for it. That is just a rumor though and as it stands, apps can only come from Apple.
There are methods for getting around this but I don't recommend them unless you feel like tinkering with your AppleTV.
64GB iPhone 5 | 64GB iPad mini | AppleTV 2 (2012) | AppleTV 2 (2010) | 2012 15" MacBook Pro, 1TB SSHD, 16GB DDR3 1600 MHz, OS X 10.8.3 Mountain Lion | Apple Lossless | iTunes AAC 192kbps VBR | iTunes 11.0.2| Library size = 1.04TB | Legacy iPods: 3G 40GB, 4G 40GB, 5G 60GB, 160GB iPod classic (2009)