GGPO (an acronym for Good Game Peace Out) is a MIT licensed (formerly proprietary) middleware designed to allow for lag reduction in online multiplayer. It was originally designed for use in arcade emulators, and remains its primary use. Specifically, it uses FinalBurn Alpha. The official site of GGPO is gone and the author instead recommends FightCade.
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.
GGPO has been open sourced in October 9th, 2019 and the source code was made available on GitHub.