Excellent emulator for Sega Genesis/Mega Drive, Sega CD and Master System Mark III for Android. Based on Genesis Plus. The source code of the emulator is open under the GPLv3 license.
6-button controller and 4-player multitap support
SVP chip support for Virtua Racing (needs a fast device)
Backup memory and save state support, auto-save and ten manual slots for save states.
Supports ROM-based games in .bin, .smd, .gen, and .sms formats, optionally in zip files
Configurable on-screen multi-touch controls & keyboard support (needs Android 2.1+ for multi-touch)
Multiplayer-capable Wiimote + Classic Controller and iControlPad support
Portrait/Landscape orientation support