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.
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.
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.