Free does not just mean gratis. There are various free licenses for art assets and source code.
Helpful lists of licenses have been assembled by free, open code and content communities:
- Debian Free Softare Guidelines compiled a practical list of licenses for both code and assets.
- The Definition of Free Cultural Works has a list of licenses, most of which can be used for game art assets.
- The Free Software Foundation lists Free Software licenses for source code and the Open Source Initiative lists open source licenses.