Bellevue (WA) - Valve’s PC game download service Steam has surpassed the 15 million account milestone, after having far and away its most successful holiday season to date.

Steam is a digital download platform that allows users to pay for full-sized PC games and then have immediate access to them. It was created by game development company Valve and has since received strong industry support from other game makers.

Over the past year, Steam really began to expand as a niche portal for specific PC games to a viable content delivery system for mainstream computer gamers. Games like Bioshock, Call of Duty 4, and Bioshock helped push the digital download service to mass reach and acceptance.

Valve is working on new features for Steam that will be available in 2008, including social networking and other community features, as well as enhanced tools for developers who want to distribute their games over the platform.

Valve noted that during the holiday season, its year-to-year sales grew 158%, and it thanked this in large part to the growth of Steam. Valve’s next major project that will be distributed over the online service is Left 4 Dead, which is coming out later this year.