Valve is particularly keen on community interaction, and it hopes to bring more gamers into the development process with a new Early Access program for Steam games. This section of the Steam library includes games that are in development but not quite finished. Users can check out the current state of each title and decide whether to effectively become beta—or alpha—testers.
You'll probably want to do a little research on the games before committing, because an early taste won't necessarily be free. In fact, you'll have to pay for all but one of the first 12 titles that are part of the program. The official FAQ notes that developers may offer early access at the regular price, at a discount, or even at a premium. I'm not sure how many folks are going to be willing to pay more for early access to unfinished games, but I can certainly see some being willing to endure beta bugs to save a few bucks.
The idea behind the Early Access program is to allow developers to engage with players, so each game has the same community hub as a full release. Players are encouraged to participate in discussions, post screenshots, and write guides. Valve notes that each developer will have a different approach to the program, so some may be more involved in responding to feedback than others.
Right now, the only widely recognizable game in the Early Access collection is Arma 3, which is billed as an alpha and costs $33 to play. (That price entitles you both to the forthcoming beta and to the full version of the game when it's done.) The other titles all come from smaller developers. Indie teams may find the program a good source of funding during development, and it will be interesting to see how many jump onboard.