• The APES
• The Glasgow Algorithms and Complexity Group 
• JChoco 
• 4C
• Edward Tsang's Constraint Programming and Optimisation Group
• Brian Hayes and his American Scientist papers
• the home of the CHOCO constraint programming system
• Claire language Forums
• 
• CSPLib
• The Constraints Archive
• Problem Reformulation and Search
• Koalog for Java solvers & configurators.
• sudoku