Xnview offers database capabilities (Categories, Rating, Tags). The interface is good, it's fast and free. But you can't write database information to IPTC/EXIF, nevertheless an integrated editor for EXIF/IPTC exists und you may copy comments from database to embeded.

I took a look a this some time ago and finally bought Acdsee Pro because I wanted a full featured DB which writes it's information also to IPTC (or in XMP-Sidercars for files without IPTC).