The steve project has developed open source tagging software for museums called steve tagger that runs on Linux, Macintosh, and Windows platforms (see the Steve Tagger 1.0 Install Guide). You can see how the tagging works at their live system site.
Here’s an excerpt from the About Steve pages that describes the project:
"Steve" is a collaborative research project exploring the potential for user-generated descriptions of the subjects of works of art to improve access to museum collections and encourage engagement with cultural content. We are a group of volunteers, primarily from art museums, who share a common interest in improving access to our collections. We are concerned about barriers to public access to online museum information. Participation in steve is open to anyone with a contribution to make to developing our collective knowledge, whether they formally represent a museum or not.
You can find out more about steve from the November 2006 "Social Tagging and Folksonomy: steve.museum and Access to Art" presentation and from other project documents on the Reference page.