The latest recipient of the Humble Weekly Sale is Croteam's hyper-realistic military simulation, Serious Sam. For a fistful of dollars, you can get the entire series, including Serious Sam's First and Second Encounters, Serious Sam II, Serious Sam 3: BFE, and the Serious Sam indie series. More seriously, all funds directed towards Croteam are being put towards development of Serious Sam 4.
For more on the contents of the sale, here's an extremely serious video:
So far, the only information we have on Serious Sam 4 is the above image and Croteam's promise that it'll be the "craziest and greatest Serious Sam game EVER." They used all-caps and everything. That's how you know they're serious. Oh god, I'll stop now.
The average price is currently hovering around the $4.25 mark, which is therefore what you'll need to pay to unlock Sams 2 & 3. The bundle runs until Thursday.