Homebrew developer Brunni has released MasterBoy for the PSP, an emulator which supports a variety of old-school handheld gaming systems, including the Game Boy, Sega Master System and Game Gear.
An interesting (and cool) feature of this emulator is that it allows you to colorize original monochrome Game Boy games. For detailed instructions refer to the PDF included in the archive.
Download MasterBoy v2.0
MasterBoy [Brunni]