I've developed a yaz php client, more to entertain myself than to put into production. It supports search and browse, uses XSL to format results pages, has expandable/contractible record entries, and demonstrates simple mashups to external sources.
Requires PHP5 and IndexData's PHP/YAZ extension, plus a bit of knowledge of configuring z39.50 clients. Tested on Fedora Core 5 only.
This application is released to the public domain -- go nuts. The only outstanding issue (i.e., where I gave up) is support for diacritics. Expect to see some mangled characters outside of English.
I'd be interested in hearing from anyone who finds it useful.
Some screen caps: