There are at least five different promos going on for various Mac shareware right now. If you're part of the Mac fold, I'd say you pretty much "gotta" check out the first two; the other three are cool, but less so.

This is part of what, in my opinion, sets the Mac experience apart from that of other platforms: there is a vibrant ecosystem of independent, small-scale development shops producing stunningly-good apps for free, or for low cost. (Although there's clearly lots of independent development going on for other platforms, there seems to be a much greater variability in the quality.) Promotion events like these give good advertising for those smaller development shops, and give Mac users the chance to try and use good software that they might never have even heard of before. I really think it's a win-win.

I've listed them below, ordered by descending perceived awesomeness.

1. MacHeist - In its second year, you act as a "spy" and solve mission-related puzzles about once a week. Free apps await after you complete each mission, as well as incremental discounts on a large bundle that will go on sale at the end. This is terrifically fun; I can't recommend it highly enough.

Coupons are available for 10% off at Best Buy (computers and some other stuff excluded) and Home Depot (coupon sent by mail, good storewide). Buy a humidifier at one place and a dehumidifier at the other, and see who wins!

Seems somewhat topical.

Register for Chick-fil-A's Million Nugget Giveaway and get a coupon for a free large Dr Pepper.  Coupon is sent in the mail in 4 to 6 weeks.