On the 7th International Conference on Chemical Structures Jeroen Kazius has a poster on finding discriminative substructures, that is, molecular fragments which can be discriminate between two acitivity classes. The software is released as Gaston, is written in C++ and has the GPL license. Later I encountered MoSS which has the same goal, but uses a different algorithm. MoSS is written in Java and uses the LGPL license.