Sonic Megamix 5.0 is a ROM hack, or fan-made modification, of a Sonic game, possibly for the Sega Mega-CD or another platform. The "Megamix" moniker hints at significant alterations and additions to the original game's content.  Version 5.0 suggests this is a further iteration on previous updates, implying ongoing development and refinement.  The original game it modifies is likely a Sonic game, featuring familiar gameplay mechanics such as speedy platforming and enemy combat.

Sonic Megamix 5.0

Sonic CD Warped V1.0.0 (USA)