Cluster101 allows you to run software on many hosts and perform a simple task scheduling. No need to install any software on the hosts (it just runs tasks via ssh). It is typically used for running scientific applications on University campuses.
Java package containing architecture to distribute CPU intensive processing tasks across the internet. A generic client will be capable of running any task the server provides it, automatically updating the code it runs.
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.