Freedom Planet is an oldschool Sonic-inspired platformer for PC from indie developer GalaxyTrail. While it's still a work in progress, a demo has been made available.
The spiky furred protagonist, corkscrew level design and robotic enemies that sheepishly run away after being defeated certainly recall Sonic, but there are major changes to the design.
Instead of simply ducking and boosting into a spin dash you must gain momentum to sprint along loops in the environment, and your velocity charges a meter in the top left corner that allows you to dash in midair ala Rocket Knight. You also have a traditional health meter unlike Sonic's one-hit-and-you-lose-all-your-rings malarkey.
Freedom Planet is still a work in progress and GalaxyTrail's Stephen "Strife" DiDuro noted that it's "aiming for a 2014 release, but I don't know quite yet" on a YouTube video for the superbly catchy soundtrack.
For now, check out the free demo that encapsulate the Green Hill zone-esque Dragon Valley's Time Attack mode.