Description:

Spin is a popular software tool that can be used for the formal
verification of distributed software systems. The tool was developed
at Bell Labs in the original Unix group of the Computing Sciences
Research Center, starting in 1980. The software has been available
freely since 1991, and continues to evolve to keep pace with new
developments in the field. In April 2002 the tool was awarded the
prestigious System Software Award for 2001 by the ACM.

http://cm.bell-labs.com/cm/cs/what/spin/spin_license.html