Sonic, Sonic Adventure, All-Stars Racing Transformed, and more can be yours.
A new Humble Bundle has just launched to celebrate the 25th anniversary of the Sonic franchise, offering up more than 10 games on PC for $10 (or less).

The games span everything from the very first Sonic up through the 3D days with Sonic Adventure and even his surprisingly excellent kart racer. You can get four games by paying just a buck, while beating the average gets you a few more (plus more to be announced). Paying at least $10 gets you every game included, while $35 includes all of that and an exclusive shirt (pictured below).

It's not a comprehensive collection of all Sonic games, but it does offer a nice collection of some of the series' better moments. The full rundown on what the bundle includes follows below.
Pay $1 or more:

Sonic the Hedgehog
Sonic Adventure DX
Sonic 3D Blast
Sonic CD

Beat the average:

Sonic Adventure 2 (with Battle DLC)
Sonic the Hedgehog 2
Sonic the Hedgehog 4: Episode 1
Sonic & Sega All-Stars Racing
More to be announced

Pay $10 or more:

Sonic Lost World
Sonic Generations (with Casino Nights DLC)
Sonic the Hedgehog 4: Episode 2
Sonic & All-Stars Racing Transformed (with Metal Sonic, Outrun, Shenmue DLC)

Note that several of the classic games, including the first two Sonics and 3D Blast--are considered "add-ons," as they're a part of the Sega Mega Drive & Genesis Classics hub on Steam. Redeeming your Humble codes for these games will allow you to access them through that collection.
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As of this writing, the beat-the-average price is actually above the $10 threshold, but that's likely a product of the bundle having only just launched. That'll likely drop under the $10 mark as time goes on.

Sega has announced plans to host a 25th anniversary fan event for Sonic in July. We don't know exactly what to expect from it, but attendees will apparently get an "exclusive first look at the future of Sonic," whatever that may be.