Final Burn Alpha 2x is a CPS 1 emulator for GP2x.
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Goomba Color is a GBC emulator written for the GBA. Even though the GBA usually natively plays GBC games, Goomba Color provides the convenience to run multiple games from a flash cartridge, and also runs on the Nintendo DS and Game Boy Micro.
jEnesiDS is a Genesis/Megadrive emulator for the Nintendo DS.
ScummVM is a program which allows you to run certain classic graphical point-and-click adventure games, provided you already have their data files. The clever part about this: ScummVM just replaces the executables shipped with the games, allowing you to play them on systems for which they were never designed!
O2Emu is an Odyssey 2 (G7000) & Videopac+ emulator for the GP2x - now including ROM selector and Virtual Keyboard.
This is a KVM to run J2ME applications on PSP.
PSX4iPhone is a Playstation 1 emulator for the iPhone and iPod Touch.
ZXDS is an emulator of Sinclair ZX Spectrum for Nintendo DS, written by Patrik Rak. It is steadily reaching a mature stage of development, and it already has some noteworthy features, like accurate emulation of Spectrum 48k, 128k and Pentagon models (including sound), or support for TAP/TZX/PZX tape files, TRD/SCL disk files and RZX playback files, not to mention the ability to download these files online over WiFi. It is available entirely for free, but any donations are always welcome.
The 'Z-Machine' is a virtual machine designed in 1979 by Infocom for playing text adventures, and has been re-implemented on nearly every computer and PDA since then.
PicoDrive is Sega Genesis/Mega Drive/Sega CD/Mega CD emulator for various ARM handhelds, currently in the hand of Notaz.