Mozilla Public License 2.0.
MPL is a reciprocal (or copyleft) licence that is interoperable and - to a certain extend - compatible with other licences, even copyleft or proprietary. OSI states it is supported by a strong community. The recipient must make the source code for any of distributed changes available under MPL, but can combine the MPL software with proprietary code, as long as the MPL code is kept in separate files.