At the time of writing, the five top sellers on Steam are DayZ, Rust, Starbound, Metal Gear Rising: Revengeance and 7 Days to Die.
With the exception of Metal Gear Rising: Revengeance -- actually a preorder rather than the game itself, which isn't available on PC until January 9 -- those best-selling titles are all unfinished games still in active development, distributed through Steam's Early Access program. And although these games are the most successful ones at present (impressively so; Starbound has sold over a million copies since release while DayZ has sold over 800,000, for example) they represent just a fraction of the developers who are benefiting from Early Access' unprecedented level of communication and interaction between developer and player.
It seems the community has very much taken to this way of doing things, then, though it's by no means a catch-all solution for every game. Story-based games, for example, would likely suffer significantly from adopting the Early Access model because it would be a lot more obvious that they weren't "complete" experiences; consequently, it's perhaps not surprising that all the most popular, successful or at least good quality Early Access offerings are either sandbox games that lend themselves to gradually implementing new gameplay modules over time, or explicitly level-based games where developers can make it abundantly clear that these levels are finished while these ones aren't.
Although the business model has grown in prominence since Steam introduced its Early Access program in March of last year, several developers have been working in this way for a while. The most successful example is, of course, Mojang's Minecraft, which sold alpha and beta access to the game well before it was as fleshed out as it is today -- and the idea of continually developing a game with the assistance of community feedback has proven so successful that even after the game shed its "beta" tag, it's continued to grow, change and evolve over time.
But Mojang's not the only developer that enjoyed the support of dedicated fans and helpful feedback from community members even before Steam implemented its Early Access program. British indie developer Mode 7 Games enjoyed enough success through selling alpha and beta access to its turn-based strategy game Frozen Synapse to make following the same model practical for its spiritual successor Frozen Endzone, for example, while French devs Amplitude Studios enlisted the assistance of its enthusiastic community in both the development of sprawling 4X strategy game Endless Space and its very different successor Dungeon of the Endless. Amplitude even goes a step further than many developers through an explicitly community-driven feature on its website called Games2Gether, where early access players are able to vote on what the team's development priorities should be -- with those who paid extra for special editions of the games in question seeing their votes carrying greater weight and influence.
Early Access can be a brilliant way of building buzz for an upcoming title, too, particularly with the huge surge in popularity of livestreaming. The team behind the frantic arcade shooter Assault Android Cactus, for example, makes use of social media to encourage and support streamers broadcasting footage of the game even in its unfinished state. It helps that the game is fun to watch and features short, snappy arcade-style levels in which players are encouraged to beat high scores and go for better rankings, of course. But perhaps the most interesting thing about Witch Beam's approach here is that getting started on promotion for the game this early and in this way will not only lead to strong sales of the PC version when it's finally finished, but will undoubtedly also help shift copies on PS4, Wii U and Vita when those versions become available, too.
As previously mentioned, however, Early Access isn't the ideal solution for all games, and some gamers have found it frustrating to see Steam (and Greenlight in particular) filling up with unfinished games that often have no clear roadmap for when a final version will become available. But for those who are interested in taking a peek behind the scenes of game development and, in many cases, having a real influence on how a game continues to evolve, change and improve over time, Early Access is one of several ways these enthusiasts can feel far more involved with their favorite creators than was ever possible under the traditional publisher model.