Super Mario Advance
a fun GBA game that combines two different games into one. Play Super Mario Bros. 2 or the original Mario Bros.
Sonic Gaiden
A modified version of Sonic the Hedgehog's Japanese rom is called Sonic Gaiden. You have the option of controlling Sonic or Mighty the Armadillo. There are only superficial differences between Mighty and Sonic.
Metal Sonic the Hedgehog
Sonic is swapped out for Metal Sonic with just a simple palette change. Sonic 1 is all that is there in this rom hack.
Sonic Tikal
a straightforward palette change in Sonic 1 that lets you play as the echidna spirit Tikal rather than Sonic
Sonic 2 Dimps
enables level selection and homing attacks against adversaries while in midair. A few new foes, some new music, and some minor changes as well.
Amy Rose in Sonic 1
For all the Amy Rose enthusiasts out there, there is a fun rom hack called Amy Rose in Sonic the Hedgehog (you know who you are). She possesses a large hammer to smash foes with in addition to being able to sprint and jump like Sonic. When she swings it, it even spits out hearts. Cute.