Arcadia: a browser-based game engine
Arcadia is an object-oriented event-driven browser-based unit-tested good ol' game engine. Arcadia exists to simplify the process of creating online games by handling tasks like user and database management, along with RPG elements like NPCs, quests, and achievements.
Hoodiequest is a free open-source browser-based role playing game with procedurally generated zones, loot, and foes. Characters collect gear by defeating enemies or shopping at vendors, and gear awards attributes that are used to determine how strong a character is in combat.
Twelve Sands is a free open-source browser-based role playing game currently in development. Based on the earlier version of the game, which was in active development for several years and hosted thousands of players.