I'm excited about being able to use Valgrind on Mac. Valgrind's "Memcheck" is much better at catching dangling-pointer bugs and heap buffer-overflow bugs than simply watching for crashes (even with MallocScribble enabled). Running a fuzzer with Memcheck can reveal exploitable memory safety bugs that would not have triggered crashes otherwise.
Update 2008-09-29: Greg Parker has released his port as a patch.
Update 2009-02-06: Valgrind developer Nicholas Nethercote has imported Greg's patch into a branch of Valgrind's SVN repository.
Update 2009-06-02: Valgrind trunk now supports Mac.