The Mana World (TMW) is a serious effort to create an innovative free and open source MMORPG. It uses 2D graphics and aims to create a large and diverse interactive world. The game aims to be a unique place for people to interact not only by fighting each other but to play life, form a community within the community, or just simply chat. Although the game is never-ending and a linear story cannot be applied, it should have enough background to describe and predefine tribes, kingdoms, and the environment overall. Fullfilled quests should lead to certain events depending on them being solved. Quests should also encourage teamwork.
Tiled is a general purpose tile map editor. It's built to be easy to use, yet it is capable of catering to a host of varying game engines, whether your game is an RPG, platformer, or Breakout clone. Tiled supports plugins to read and write map formats other than its own map format.
Oligopoly is a Monopoly-like game as a Java applet, directly playable from the Internet. It's written in Java and the object design is very true to the game. It should be easy to modify. It comes with nice graphics and is quite fun, although it lacks many features of the original game. It is currently only in German.
X-Plane is a flight simulator that reads in the geometric shape of any aircraft and then figures out how that aircraft will fly. It does this via an engineering process called "blade element theory", which involves breaking the aircraft down into many small elements and then finding the forces on each little element many times per second. These forces are then converted into accelerations, which are then integrated to velocities and positions. This gives X-Plane the most realistic flight model available for personal computers.