Stella is on of the best emulator of Atari 2600 game console, running on many different systems, such as Linux, Solaris, Windows, MacOS/X, WinCE, OS/2, GP2X.
It has been written initially by Bradford Mott, see Stella site for details.
GP2X-2600 is a port on GP2X of ZX-81’s previous port to PSP. ZX-81’s PSP version is based on the work of Aenea who was the first to port Stella to PSP, and David Voswinkel who’s now in charge of the PSP port in Stella team.