there are a few. The 3 major ones are:
-KisMac (only works well with some 802.11 cards, but I think it's the most powerful)
-iStumbler
-MacStumbler
No matter what you use it probably won't be easy to crack the pass. If you have a card that kismac likes (or even better, 2 cards that kismac likes; at least one in the PCMCIA slot if you have one on your laptop), then kismac will probably be the best one to crack the pass. Kismac will work on a basic level with airport extreme cards, but it doesn't like them very much and won't work with all the features.