ERP is one of the most complicated types of software and typically requires a massive company to support it. I doubt there's a large market for off-the-shelf, DIY ERP. However it might make sense to pick off one of its components and target it to small business, for example an app that keeps track of personnel.
Here's a more complete description: http://en.wikipedia.org/wiki/Enterprise_resource_planning Evaluation-matrix might want to use a different system, too. It's currently down.
