Build Your Own Net Dream

BYOND (Build Your Own Net Dream) is a free Game Engine for 2D tile-based multiplayer games. It was originally created to make MUDs, but eventually grew to include graphics. While it's not that popular on the broader internet, several individual games are.

Notable BYOND Games:

 * Mitadake High - A role-playing game with a Ten Little Murder Victims premise. It was popular enough to spawn it's own webcomic, Mitadake Saga.
 * Nestalgia - It's an RPG in the vein of Dragon Warrior. One of the most popular games on BYOND.
 * Space Station 13 - Became so popular on the Something Awful forums that it was literally decompiled and pieced back together by a goon. There are three main versions now, OpenSS13, Baystation, and the ever-nonsensical Goonstation.
 * Welcome to Utopia - A role-playing game based on the Manga Shitsurakuen.