Actually, just writing the tests is often a damn fine way of finding bugs. No exactly what OP is after at the moment, but something that is at the forefront of my mind because I'm in the middle of writing a set of tests (in Perl :) for some XML processing C++ code and turning up a pile of bugs as I go.
However it does suggest another test avenue: write test harnesses for modules so that you can exercise them in isolation and better understand how they work. If the test harness ends up part of a regression test system so much the better.
DWIM is Perl's answer to Gödel
