Mac OS X: The 2G Apple TV was jailbroken shortly after its release, but until now there hasn't been a simple jailbreaking method. That's all changed with Seas0nPass.
Seas0nPass is a jailbreaking app for your 2G Apple TV that's similar in functionality to Pwnage Tool (the simple jailbreaking tool for other iOS devices). Seas0nPass has two functions: it can create a jailbroken IPSW file that you can use to "restore" your 2G Apple TV or it can boot your jailbroken device. Really, the interface is that simple:
Yes, this does mean Seas0nPass is a tethered jailbreak, so you may want to wait for an untethered option, but you may want to consider how often you actually have to boot your Apple TV. It's also a minor inconvenience since the Apple TV isn't going anywhere and, presumably, you can just plug it in to your computer and quickly boot it if necessary. A tethered jailbreak with an Apple TV isn't quite as risky as an untethered jailbreak with a mobile iOS device.
If you want to proceed, the remaining steps are simple and Seas0nPass will walk you through each one. Just like any other iOS device, you put it into DFU mode (in the case of the Apple TV, this is done by holding the MENU and PLAY/PAUSE buttons down for seven seconds) and restore the device and restore using the created IPSW. What's especially nice about Seas0nPass, however, is that it will pretty much automate most of this process for you. It'll even download the latest IPSW it needs to patch so you don't have to find it yourself or worry about grabbing the wrong version.
Once you've successfully completed the jailbreak, you just need to boot tethered and you're done! So what can you do with a jailbroken Apple TV? You can install Plex and incomplete-but-functional versions of Last.fm and a web browser called Couch Surfer.