GOG.com has launched a Games in Development program to give gamers early access to games that aren't quite finished yet. This is similar to Steam's Early Access program, but GOG is taking a slightly more curated approach.
Right now, the five titles you have early access to are Starbound, Ashes of the Singularity, Project Zomboid, TerraTech, and The Curious Expedition. All of the five games are available on Steam's Early Access program, too.
You can turn off auto-updates and go back to a previous version of the game you're playing with GOG's optional Galaxy launcher client. If you're playing the early access games that are still in a state of flux, that's a good option to have. Like everything GOG offers, one big area where they differ from Steam is the lack of digital rights management.
According to the announcement, GOG says that they will be picky with the early access games: "We're hand-picking only the games we can truly stand behind." Their plan is to offer what they decide are the "most promising titles" as well as games people have requested on the Community Wishlist.
And if you try one of the games still in development and hate it, no worries - just return it within the first 14 days of purchase. It doesn't matter if there were technical glitches or it wasn't what you expected, the refund policy is no questions asked. So what are you waiting for?