Joystiq and Massively have 2,000 Steam keys for Nether to give away, coinciding with the release of the new update. The Steam passes include closed beta access, 50 Nether gold, a Chosen-only exclusive in-game item, five 72-hour guest keys for your mates, forum access, and a highlighted forum name and rank for your account. Additionally, once the Steam bundles are gone, we've got 18,000 more guest keys that grant 72 hours of beta access without the other goodies.
To redeem your Nether Steam key, install and launch the Steam client software and log into your Steam account (the client, not the Steam website!). Click the Games Menu, then Choose Activate a Product on Steam, and then follow the onscreen instructions to complete the process. You and your friends can redeem your guest keys by creating an account on the Nether website and following the instructions therein to download the game. Steam is required for all keys.
Have fun saving the world!
Update: We're already out of Steam keys, but there are still guest keys left.