a solver for the Su Doku problems which appear in various newspapers and seem to be a current craze.
I've yet to find a solvable problem which it can't do (not that I've looked very hard). It warns when a puzzle has multiple solutions, and contains a 'hint mode' allowing you to check your own attempts without peeking, too. also available to download is a (slightly larger) un-mangled BASIC version, so you can look at the source-code if you want (the solving routine is horribly messy, however, as I've added quite a lot of features higgeldy-piggedly - sorry!).