as seen at freshmeat: "This release adds backwards-incompatible API modifications, the ability to write to databases, an API for writing to databases, a QuickStart tutorial, Doxygen use for automatic documentation, prototype distributed searching, and multiple bug fixes. There are now no test suite failures." Visit open.muscat.com for more.
Both the Ecila (French) search engine codebase, catalog-1.0, and the U.K. eLib endproduct ROADS v2+ 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). Open source as "exit strategy" isn't terrifically sustainable, but a step in the right direction nonetheless.
from the DBA Sciences page: "Database Advisor (DBA) was created to aid database users in selecting the best database for their query. DBA spawns a search process for each database vendor, and returns the hits on the query to the user. It sorts these results so the user can see where each database stands relative to the others." DBA is GPL'd, and its components are all free according to one license or another. You can even take it for a test drive...
the Simon Fraser University Library Research Instrument (SLRI) is "a web to Z39.50 client interface" brought to you by the good folks at SFU. it's an adaptation of the web to Z39.50 gateway developed by Harold Finkbeiner at Stanford, licensed under GPL and recently spied at sourceforge.net as well.