Steam needs 300 beta testers for the SteamBox, offically called the Steam Machine.

Will I be able to build my own box to run SteamOS?
Yes.

Can I hack this box? Run another OS? Change the hardware? Install my own software? Use it to build a robot?
Sure.

What games will be available during the beta?
The nearly 3,000 games on Steam. Hundreds already running natively on the SteamOS, with more to come. The rest will work seamlessly via in-home streaming.
Yes please. Apply here: http://store.steampowered.com/livingroom/SteamMachines/