IMATCH at
photools.com - Solutions for Image Management and Digital Photography. is a very comprehenive photo database that provides extensive meta data editing capabilities and macro development tools. I happen to be a smugmug user and I noticed recently that an API interface for IMATCH has been developed. It does have a learning curve that may not be worth the effort if all you want is a library listing of thumbs.
The tool has a pretty big following, i.e. discussion groups, and I've always received good support from them. Finally, it is in its Nth revision and I consider it a mature well tested product.