Source development management

My brother is working on a project:

http://www.scirra.com/

It's getting fairly near completion, however he is having trouble managing the source code with his fellow developers.

He's tried just doing it over msn, but they loose files every now and then and it can get confusing.  They also tried folder share, but it uploads temp 10MB files created by compiling the project.

Does anyone know of a good free source management program?