I’ve been maintaining a publicly-editable wiki, museum-api.pbwiki.com since early 2009. The impetus was the development of an API for a subset of the Science Museum’s collections. I introduced it with a post on Open Objects, Get thee to a wiki – the great API challenge in action, saying:
Help us work on an informal, lightweight way of devising shared data, API standards for museum and cultural heritage organisations – museum-api.pbwiki.com is open for business.
These days, the focus is on collating a list of cultural heritage APIs and open and/or linked datasets from museums, libraries, galleries, archives etc and collecting examples of things made with cultural and historical data, with the core goal of cultural heritage organisations make content re-usable and helping programmers access cultural and historic content.