GGPO (an acronym for Good Game Peace Out) is an open source middleware developed by Tony Cannon in 2006. It's designed to hide the appearance of lag in online multiplayer, and does so using a technique called rollback networking. It was first implemented as a web service using FinalBurn Alpha but, because it distributed ROMs to players, it didn't last very long. Cannon used this site for two purposes; to adjust the code according to feedback from players, and to market GGPO to game developers. Eventually, it was incorporated into games like Skullgirls and Street Fighter III: 3rd Strike Online Edition. While the GGPO service doesn't exist anymore, alternatives exist; Cannon recommends using FightCade specifically.
GGPO is available under the MIT license and is available on GitHub.