Freeware by Chad Whipkey and Theodore Hwa. Win 95/NT.
Program for solving and composing retrograde chess problems. In retrograde analysis the task is to deduce which moves lead to a given position - in a sense you play backwards. Sample problems (a lot!) can be found at The Retrograde Analysis Corner.
Download from http://xenon.stanford.edu/~hwatheod/Retractor/ (157 KB).