Desura is an online games platform from where you can download, purchase and play games similar to the hugely popular Steam platform.
In Desura you'll find some free to play games plus some mods to popular games like Half-Life and Red Alert. Unfortunately however, you won't find many major titles on there - it's very limited when compared to Steam.
Desura does however also provide tools and hosting for game developers, as well as a useful forum for the gaming community to exchange opinions and tips. Desura is free to register for and within a few clicks you will be able to play and download games with the rest of the Desura community.
Desura is an interesting alternative to Steam although it lacks many really big games.