The Crysis series supports a variety of mod tools, all supplied for free from Crytek.
Crysis[]
Crytek selected two modification teams to work with a Crysis SDK before the game's release. These mod teams received Crytek support during development and the mods were released shortly after Crysis was released. The game comes released with the Sandbox 2 editor - an extremely versatile program that can be used to, in effect, make a whole new game.
Crysis Warhead[]
For Crysis Warhead, Crytek released a newer version of the SDK for Crysis Wars. However, the new SDK must be downloaded as it comes with Crysis Wars patch 1.4.
Crysis 2[]
Crytek released the CryENGINE 3 SKD editor for Crysis 2 shortly after the game's release. The SDK retains many of the same features found in previous versions.