RDKit-based implementation of the method described in:

Natural Product-likeness Score and Its Application for Prioritization of Compound Libraries 
Peter Ertl, Silvio Roggo, and Ansgar Schuffenhauer
Journal of Chemical Information and Modeling, 48, 68-74 (2008)
http://pubs.acs.org/doi/abs/10.1021/ci700286x

Contribution from Peter Ertl