Both the Ecila [1] (French) search engine codebase, catalog-1.0 [2], and the U.K. eLib endproduct ROADS v2+ [3] are open source and increasingly used tools for building web-based catalogs a la Yahoo. Some eLib folks have explicitly turned to open source as a way to keep formerly well-funded projects going (see the press release describing this decision [4]). Open source as "exit strategy" isn't terrifically sustainable, but a step in the right direction nonetheless.
Links:
[1] http://www.ecila.fr/
[2] http://www.senga.org/Catalog/html/
[3] http://www.ilrt.bris.ac.uk/roads/
[4] http://www.ilrt.bris.ac.uk/roads/news/latest/futures/