There are thousands of open source projects globally and a good place to find them is via Source Forge
Here are a few that peaked my interest.
The RoboCup Soccer Simulator – The RoboCup Soccer Simulator is a research and educational tool for multi-agent systems and artificial intelligence. It enables for two teams of 11 simulated autonomous robotic players to play soccer (football).
Brain workshop – Brain Workshop is a Python implementation of the Dual N-Back mental exercise. This exercise is the only mental activity that has been scientifically shown to improve your short-term memory (working memory) and fluid intelligence.
View your mind – is a tool to generate and manipulate maps which show your thoughts. Such maps can help you to improve your creativity and effectivity. You can use them for time management, to organize tasks, to get an overview over complex contexts
Sweet Home 3D – Sweet Home 3D is an interior design Java application for quickly choosing and placing furniture on a house 2D plan drawn by the end-user, with a 3D preview.
What are the ones that you find interesting?