SEGA has Sonic Dash (iTunes Link) for the iPhone and iPod Touch in the iTunes App Store for FREE (regularly $1.99) for a limited time.

Play as Sonic the Hedgehog as you dash, jump and spin your way across stunning 3D environments. Swipe your way over and under challenging obstacles in this fast and frenzied endless running game for iPad, iPad mini, iPhone & iPod touch. Comes with 10 free revives in-game now for a limited time.