This video, from the Boxee blog, shows how you can use a USB memory stick to install the open source social media center. 

Boxee plays media from your computer and other devices in your home network, and can connect you to various Internet sources that allow you to stream or download movies, tv shows, music and photos. Versions are available for Mac, Windows, Linux and Apple TV.

how to install boxee on your Apple TV:

1. download the ATVUSB-Creator from http://code.google.com/p/atvusb-creator/
2. insert a “bootable” USB drive into your Mac (Windows and Linux versions coming soon)
3. run the ATVUSB-Creator and select which plug-ins you’d like to install
4. remove the USB drive and plug it into your Apple TV
5. power on your Apple TV and the patchstick will run the ATV bootloader
6. after the bootloader finishes, remove it and restart your Apple TV
7. it now has options for Boxee / XBMC on the main menu
8. click on Boxee, then select update (this will download boxee from the Internet)
9. once boxee is done installing, restart your Apple TV
10. disable your Apple TV screensaver (set it to ‘never’)
11. click “Boxee”, then select “Boxee” in the sub-menu to start boxee.