Sonic 25th anniversary bundle is up. Worth noting that this bundle doesn't provide one key per game, but one key per tier.

For $1+:
Sonic the Hedgehog
Sonic Adventure DX
Sonic 3D Blast
Sonic CD

For average:
Sonic the Hedgehog 4 - Episode I
Sonic & SEGA All-Stars Racing
Sonic the Hedgehog 2
Sonic Adventure 2 + Battle DLC
This tier will have bonus games.

For $10+:
Sonic Lost World
Sonic Generations + Casino Nights DLC
Sonic the Hedgehog 4 - Episode 2
Sonic & All-Stars Racing Transformed + Metal Sonic & Outrun and Ryo Hazuki DLC

For $35+:
Exclusive Sonic 25th Anniversary Shirt

Free: Sampler pack for Archie Sonic comics, containing:
-Sonic The Hedgehog Vol. 1: Countdown to Chaos
-Sonic Select Vol. 9: The Games
-Sonic The Hedgehog Vol. 2: The Chase.

All tiers come with a 10% off Humble Monthly for New Subscribers coupon.