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.
- 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.
- MacUpdate Bundle – Bundle of 7 applications on sale over the next week or so at roughly 90% off the total retail price. Three additional apps will “unlock” and become part of the bundle if sales are high enough.
- MacSanta – Several apps listed each day from now through Christmas at 20% off. Apps from previous days available at 10% off. Lots of variety here, but the discount isn't as good as the previous two.
- Give Good Food to your Mac – Piece together your own bundle of apps, with a discount percentage that increases based on how many apps you buy. I didn't find any of the apps particularly compelling, but you might disagree. Ends 12/10.
- TheMacPak Santa's Stocking – A bundle of five apps and two games at a pretty steep discount, available through 12/10. The only problem: you don't know what the apps are before you buy them!