Steam has launched a new programmed called “Early Access”, where Steam community involved games are available to play while in Beta. You can either play a free game, or purchase one with money. This is not the same as a pre purchase, with Early Access when you buy the game you get full access to it in its current playable state. Discretion is encouraged on purchasing a game that is in Beta.
There are currently 12 games in this program, including AMRA 3 Alpha and Under the Ocean. One should check the previous updates to games to make sure your purchase will be well worth it, and official release dates of games are completely up to the developer. For more detailed information on Early Access, check out the Early Access FAQ.