Alan_quatermain has just posted the AwkwardTV Loader, a plug-in that displays several available third-party plug-ins on Apple TV.
All these programs can be downloaded directly from the Web and then installed on the fly. The YouTube video linked here, and shown at the top of this post, depicts this process.
Right now there's a beta, but the beta is closed. Yet that always won't be the case. The AwkwardTV site says potential testers can keep an eye out for information on the #awkwardtv channel or the wiki.
For now, the plug-ins (with descriptions offered by AwkwardTV) include:
Simple file browser plugin
Wakes up remote media servers
Apple TV Closer
Quit Apple TV interface with remote control
AppleTV RSS Plugin
RSS Feed Reader
Display sports scores
Gives you a weather report
Run any perl script
The swiss army-knife of QuickTime® components
Automatic mounting of shares on bootup.