Dan B. Cauthron lists 150 services which provide print-on-demand production.

Science Fiction and Fantasy Writers of America (but not limited to these genres!) explores Print-On-Demand pros and cons, service sales statistics, POS issues to consider, and electronic self-publishing discussion. 

Clea Saal’s Books and Tales provides comparisons of a number of POD services, as well as a series of articles on the stages of the POD self-publication process

Dehanna Bailee’s Print-on-Demand Database offers side-by-side comparisons.

Novelist Piers Anthony maintains an internet publishing resource that lists and describes electronic and POD-based publishers, and flags those that have problems or are the focus of complaints.