"Gotham City Imposters," released earlier this year, has gone free-to-play on Valve's Steam service.

The free-to-play online game market is large and rapidly expanding. Millions of users engage in free-to-play titles every day, ranking up characters as they engage with others and improving performance through in-game Microtransactions. Following in the footsteps of Sony Online Entertainment's move to offer a free-to-play subscription to "DC Universe Online" late last year, Warner Bros. Interactive Entertainment has announced "Gotham City Impostors," released earlier this year, is following suit on the PC.

While the Xbox Live Arcade and PlayStation Network versions of the team-based first-person shooter will remain at premium, online users of Valve's Steam and Steamworks service will be able to download the game at no charge, starting now. Following the announcement prior to this weekend's PAX Prime event in Seattle, the company provided a number of kiosks for players to check out the game in action.

The full "Gotham City Impostors" game will be included in the download, along with all additional downloadable content packs, which includes a number of weapons, mascots and maps, including such familiar locales as the Ace Chemical Plant and Gotham Power Company.

In addition, Warner Bros. has also included join-in-progress matchmaking, which will make sure that users logging in for the first time can find populated matches without having to wait too long -- a huge benefit for free-to-play games.

Microtransactions weren't discussed in Warner Bros.' announcement, but judging by the free-to-play version, there really aren't many. All the weapons and gadgets -- including the Glider Rig and Grappling Hook -- are available in the core game, as well as various costume options for both the Batmen and the Jokerz, the two teams up against each other in "Gotham City Imposters."

"Gotham City Imposters" is currently available for free on Valve's Steam platform.