I'm not a huge fan of spending a lot of money on apps for various and sometimes illogical reasons. I am going to list a few of the place I check first for either deeply discount or free Mac apps. Please post a few places you check if you have any. Enjoy!
I have absolutely no monetary arrangement with the following entities and do not benefit in the slightest from there traffic. The sole mission of this talk is to help Mac users find the best prices on apps.
http://twodollartues.com - Two Dollar Tuesday promotions on the Mac App Store
http://www.appbuzzer.com - AppBuzzer has free and discounted Mac apps.
http://www.mupromo.com - MacUpdate Promo often has deeply discounted Mac apps as well as app bundles and discounts.