Calames is a recently released catalog of archives and manuscripts in French universities and national institutions. It is provided by Abes, which also provides Sudoc the French universities union catalog.
Not only is there a shared public catalog, but Calames supports a shared cataloging environment for the creation of finding aids.
Why have a shared cataloging environment for materials that are unique? The rationale is straightforward: shared cataloging rules, a shared working environment, mutual assistance in a shared network, and common access to authority files and other tools, all support coherence in the union catalog.
For a description of the cataloging environment see the article on pages 18-19 of the February-March 2008 issue [pdf] of Arabesques, the magazine of Abes.