Cube Platform is a decentralized gridcomputing system that uses P2P Pastry protocol for communication between nodes. It's a big data storage written in Java.
GridPlan is an open source capacity planning and simulation tool for GridComputing. GridPlan estimates the cost and performance of Grids comprised of Processor, Storage, Router and Switching nodes.
JCGrid is an easy-to-use tools for real-world grid-computing. It requires only to install Java Runtime and you can easy split your cpu-intensive task among several Workstations/PC/Mac/etc. It includes support for POVRay and MandelGrid as usage examples.