Sonic Generations (along with Viking: Battle for Asgard) have been unlocked for the beat-the-average tier, which currently sits at a mere $3.72.
Generations is a solid port. Also, be sure to check out the excellent Unleashed Project mod too!
As mentioned before, if you already bought the dollar tier, you can upgrade your purchase through the bundle key page Humble sent you. And if you already beat the average, then the extra games should be waiting for you on that page too.
Go grab it here!