Start64!Gecode is an open, free, portable, accessible, and efficient environment for developing constraint-based systems and applications.
Gecode is:

open

Gecode is radically open for programming: it can be easily interfaced to other systems. It supports the programming of new propagators (as implementation of constraints), branching strategies, and search engines. New variable domains can be programmed at the same level of efficiency as finite domain and integer set variables that come predefined with Gecode.

free

Gecode is distributed under the MIT license and is listed as free software by the FSF. All of its parts including documentation, implementations of global constraints, and examples are available as source code for download.

portable

Gecode is implemented in C++ that carefully follows the C++ standard. It can be compiled with modern C++ compilers and runs on a wide range of machines (including 64bit machines).

accessible

Gecode comes with both tutorial and complete reference documentation that allows to focus on different modeling and programming tasks with Gecode.

efficient

Gecode offers excellent performance with respect to both runtime and memory usage. A comparison with some other systems is available.