Interesting evaluation of Z39.50.
Sandra D. Payette and Oya Y. Rieger from Cornell University discuss Z39.50 with a user-centric approach.
The following pages on Z39.50 activities in Europe are being built up gradually. The systems and services listed are not therefore comprehensive. There is currently a UK bias, but coverage of other European countries will be more substantial in time.
US Library of Congress Maintenance Agency page for International Standard Z39.50. I think this is a close to an official homepgae one can get. Lots of stuff there.