If you've already bought Bundle 2, you're free to download Bundle 1, which has World of Goo, Aquaria, Gish, Lugaru HD, Penumbra Overture, and Samorost 2, all playable on PC, Mac, and Linux. If you haven't bought Bundle 2 yet, you're only eligible to get the bonus games if you spend more than the average payment of $7.34. You have through Christmas Day to pick up this year's offerings.
You can download these straight from the site, but you can also unlock them in Steam (by getting a keys from the HIB site), with a few caveats. The Steam keys are "a little glitchy," lacking access to Samorost 2 and lacking Mac versions of Gish and Aquaria. In addition, organizer Wolfire only has 150,000 keys to hand out, so if you already have the games on Steam, or you can deal with just downloading the games (DRM-free) and not having that Steam integration, perhaps you could leave the Steam key for someone else.