I had the opportunity during my PhD to participate in the development of the solver Mistral-2.0 in different stages. I was mostly involved in the MiniZinc Challenge entries.

Mistral is a C++ library under the GNU General Public License and is hosted on github . It implements most arithmetic and logic primitive constraints as well as some global constraints (AllDifferent, Linear Equalities, Element, etc...). Standard search strategies and heuristics are also implemented (Restarts, Weighted Degree, Activity Based Search).
More Information can be found here.

Hybrid-Mistral is a Hybrid CP/SAT Solver dedicated to resolve Disjunctive Scheduling Problems. The SAT Engine implements standard SAT techniques such as Adaptive Branching (VSIDS), 2-Watched Literals, 1-UIP Cuts, etc. CDCL (Conflict-Driven Clause Learning) features are implemented on top of Mistral-2.0. Propagators are now augmented with explanation routines that are called only during Conflict Analysis.