Underworld is a massive multiplayer online role playing game (MMORPG) based on web technologies, so it can be played with any web browser. It is a fantasy game in which the player can be either a human or a monster: combats, spells, buy/sell items
Tactical Warfare Engine is an Open Source 2.5D Real-Time Strategy game engine made using OpenGL. It aims to be a replacement for the aging StarCraft game engine, but also supports the creation of entirely new RTS games.