JesTer (for Java code and JUnit tests) (free, open source) ("strong" mutation testing) - jester.sourceforge.net
PesTer (for Python code and PyUnit? tests) (free, open source) ("strong" mutation testing) - jester.sourceforge.net
insure++ (for C++ code) (commercial product) - www.parasoft.com/products/insure/papers/tech_mut.htm
GCT does mutation coverage for C ("weak" mutation testing). (open source, standard distribution hasn't been ported to modern versions of Unix or Windows) - www.testing.com/tools.html
The original Mothra tool is available - ise.gmu.edu/faculty/ofut/rsrch/mut.html
MutationTestingTools