Check out Simple Bots at Instructables. Randy Sarafan, author of 62 Projects to Make with a Dead Computer, presents an array of robots using these guidelines: Easy and quick to build, simple, mobile, autonomous, and eventually able to incorporate more complex controls and/or be assimilated by a more complex system. The results are fascinating and include common household objects such as telephone handsets, paint brushes and small plastic containers. Simple bots typically rely on one or more RC servomotors modified for continuous drive for their mobility along with a pack of AA batteries.