PicoDrive is an open-source Genesis/Sega (Mega) CD/32X/Pico emulator. It was originally developed for the UIQ2, before moving onto the GP2X, PSP, and other handheld devices.
The most actively developed version was for the GP2X, as well as being the best port due to higher levels of optimization.
PicoDrive is also available as a libretro core for RetroArch.
notaz provided a massive update to bring the libretro core up to date with upstream. This update included many accuracy improvements, along with some nice features like 68k overclocking.