GGPO (Good Game Peace Out) is a proprietary middleware designed to allow for near-lagless multiplayer. It was originally designed for use in arcade emulators, and it still currently its primary use. Specifically, it uses Final Burn Alpha. It has also been licensed by video game developers for use in their games, such as Skullgirls and Street Fighter III: 3rd Strike Online Edition.
RetroArch uses a similar system. However, the two are unrelated.
Latest version (requires Adobe AIR)