via http://www.aep-emu.de/

Marat Fayzullin has ported his (commercial) MasterSystem, GameGear, SG-1000, SC-3000 and SF-7000 emulator MasterGear to the Android platform: MasterGear for Android

Quote:
New in 2.6.10:
  • Added new portrait mode virtual joystick.
  • Added configurable custom key mappings.
  • Added optional haptic feedback.
  • Application now defaults to "auto" screen orientation.
  • Finally fixed Samsung Galaxy S2 issues.