Minion is a fast constraint solver. It can solve any problem which can be expressed as a list of variables and a list of constraints, for example, Sudoku. It has been used to solve problems with thousands of variables and constraints.
Minion is a solver for constraint satisfaction problems. Unlike constraint programming toolkits, which expect users to write programs in a traditional programming language like C++, Java or Prolog, Minion takes a text file which specifies the problem, and solves using only this. This makes using Minion much simpler, at the cost of much less customisation.