It's always pretty cool when game companies get their players involved in the game-making system. Even if you have a couple dozen playtesters for a game, there's always the chance that something can be missed. So letting a rules set go to a public beta can make it so thousands of people look over the rules. Catalyst Game Labs has done that for Combat Manual: Mercenaries, the first in a new line of sourcebooks for Alpha Strike.
Players that want to check it out can pick up the .pdf for $5. If you don't already have the Alpha Strike rulebook, they're having special bundle deals for that .pdf as well. So if you've wanted to get in on the game, but haven't done so yet, you can still be part of the open beta. If you do purchase the Beta .pdf, you'll automatically get upgraded to the final version when that one eventually goes live (so it's almost like a pre-sale, if you want to see it that way).
The Open Beta will last until the 10th of December.