Tails in Sonic the Hedgehog (aka Tails in Sonic 1), is a hack of Sonic the Hedgehog for the Sega Mega Drive/Genesis by drx featuring Tails as a playable character instead of Sonic. Tails can fly, Spin Dash and swim in the same way that he can in Sonic 3.
Tails in Sonic the Hedgehog Bugfix
On June 8, 2007, Puto released an update to Tails in Sonic the Hedgehog, which featured several improvements over the original hack: bugfixes, a corrected palette, and Sonic 2-styled tired and swimming sprites. More bugfixes have been released since, and on 12/31/07, he released version 2.0, completely rewritten from scratch. Version 2.1 was released on 10/09/08 to no fanfare; its purpose was to fix some differences in Tails' flight behaviour, making it 100% identical to the flight behaviour used in Sonic 3. Version 2.1.1 was released on the next day to fix one more bug.
Release thread at Sonic Retro
Bugfix release thread at Sonic Retro
Version 2 release thread at Sonic Retro
Version 2 release thread at SSRG