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.
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.