C++

Free Integrated Development Environment (IDE) for C and C++

Windows
Dev-C++  is a free full-featured IDE distributed under the GNU General Public License for programming in C/C++. [Dev-C++, Wikipedia]

Linux and MAC
Eclipse CDT* is a fully functional C/C++ IDE based on the Eclipse platform. [CDT wiki page]
* CDT  (C/C++ Development Tooling)