PlexConnect is a clever project: a free command-line tool that lets you view content from your Plex Media Server on your Apple TV--without jailbreaking Apple's set-top box. That scratches an itch for me; though I maintain a Mac mini media center, the Apple TV runs quieter and cooler, and generally just works. (And on the rare occasion it doesn't, I don't have to fumble around for a mouse and keyboard to troubleshoot it.)
A caveat: If you're Terminal-shy, then PlexConnect's probably not for you. While it doesn't require a lot of command-line expertise, those who have never ventured into that territory may find it a hassle. There are plans to at some point package the software into a friendlier Mac app--it's part of an ambitious road map--but for now, it's go command-line or go home.
PlexConnect does its magic by running two small servers on one of your local Macs: a Web server and a Domain Name System (DNS) server. It's the latter that handles the magical part; you configure your Apple TV to look towards this Mac for its DNS server, and PlexConnect intercepts requests for the URL http://trailers.apple.com and redirects them to its Web server, where it provides a custom version of the Plex interface reminiscent of other Apple TV apps, like Netflix and Hulu. (DNS requests to other addresses, meanwhile, should to be passed through to your normal DNS server, thereby maintaining the rest of your Apple TV's normal operation.)
On your Apple TV, you can browse or search your Plex content with your remote, as you would any other Apple TV function. Some content isn't currently available, such as photos from Aperture and iPhoto, and there are bugs here and there. PlexConnect offers only a few configuration options on your Apple TV--you can choose whether you view your listings of movies, TV shows, and TV seasons in list or grid format, as well as tweak some transcoding preferences for video quality and subtitle size, along with some more technical options.
In a word, it's nothing short of brilliant. But, in my experiences setting up the software, I did run into a few minor issues. The most obvious is that running PlexConnect will hijack your Trailers app, so if you want to watch movie previews from Apple's site, you'll need to either disable PlexConnect or find another solution.
One limitation of the PlexConnect software itself is that it doesn't yet fully support third-party video and audio channels on the Apple TV. The project's FAQ notes that some channels, though, do work; I went through several and found only a couple that actually let me play back media, including Funny or Die and PBS. The rest I tried currently fail with a variety of errors before you can get to the actual video.
At one point--I haven't managed to figure out exactly why--PlexConnect seemed to interfere with AirPlay when I was trying to play videos from my iPad mini on my Apple TV; all I got was a constantly spinning loading indicator. Suspecting that PlexConnect might be the culprit, I shut the software down and AirPlay returned to its normal working condition. I've not been able to duplicate that in subsequent tests, though, so it's possible it was an issue with my iPad.
Admittedly, PlexConnect is a niche project, but to those who've been looking for a native interface to their non-Apple media on their Apple TV and don't mind fiddling with some more technical details, it's definitely worth a look.