After the Directx 11 upgrade, Crytek promised the modding tools that will follow the planned update. Today, as promised, Crytek has released modding tools for Crysis 2, which among other things include Sandbox 3 Editor, FMOD Designer, Poly Bump, and plugins for Maya and 3DS Max.
Sandbox 3 editor is for creation of custom maps and mods. The package also includes custom samples for the guide. What are you waiting for ? Go Download it and start modding.