I am currently working on the SRU, MODS, and MARC stuff for Sinciput. (Translation: I am writing an interface to the Library of Congress). In so doing, I am trying to hunt down Java projects that might provide some of the base functionality. It's looking like I am going to have to write my own MODS library, which I find surprising. But it will be a good opportunity to let the construction of XElement shine through.
Here are some interesting projects:
MARC4J provides a Java SDK for MARC records. It can convert to and from MODS data using XSLT. Less than ideal, but maybe OK: http://marc4j.tigris.org/
Mike Taylor's CQL library:
JZKit, a Z39.50 toolkit in Java. The lack of documentation is frustrating:
IndexData provides PHP, C++, and Perl libraries:
ZOOM Z39.50 Bindings:
Cheshire at Berkeley is a Python ZiNG-ish set of libraries:
RefBase is a bibliography database in PHP:
I guess I also ought to find out about MIT's DSpace. I keep hearing about it.