The source code for Enemy Territory is now available for download from FilePlanet so you can start your modding. Enemy Territory is a free multiplayer expansion for Return to Castle Wolfenstein. Gamespy summerizes it as:

"Enemy Territory challenges gamers to the ultimate test of teamwork and strategy. Each of the five character classes is critical to a team’s ultimate victory or defeat on the battlefield. The Covert Operative class allows players to steal uniforms, perform reconnaissance and gain access to enemy positions. While, the Engineer allows the Axis and Allied teams to lay and diffuse mines as well as build battlefield bridges, towers, forward command bases and other improvements in the midst of combat to gain advantages for their team. "