Concordion is an open source tool for automating Specification by Example. The specifications are so readable they can double up as system documentation.
Concordion specifications are written in HTML, so can be easily hyperlinked into a navigable structure. The specifications contain examples of behaviour which, when automated and executed against the system, are colour-coded to show the current state of the system. By executing these frequently, you can be confident the documentation is always up-to-date.