Depends on what kind of programs you're looking for.
MU and MacHeist offer cool gadgety kinds of things. If you're looking for savings on big-name software, start at Amazon. They've recently had combos of rebates and price cuts that put things like VMWare Fusion 2 as low as $5 (required a cross-grade rebate). Amazon's prices can shift in the breeze (to come in just under some other retailer's sale at that moment), so if you see a price you like, snap it up - if the other retailer goes off-sale, then the price will go back up at Amazon too.