Moodle is an open source virtual learning environment widely used in education, enjoying widespread, and increasing adoption. Some time ago, Moodle support was announced by Caucho - http://www.caucho.com/news/press/2008-11-04/ [^] - though perhaps this claim is no longer maintained.
In a development environment, I have successfully had Moodle 1.9.7 running on Resin 4.0.2 with a Postgres 8.4 database, though this environment has only seen limited action. There are particular issues with a "for real" deployment of Moodle. The first, Nam has already reported on my behalf: bug 2999 regarding the lack of LDAP module support in Quercus. Other modules that could be usefully implemented are the openssl and xmlrpc extensions, which are used in Moodle networking. (This feature is not terribly widely used in education as far as I know). Additionally, the soap extension should be supported, as this would allow using Moodle with a Shibboleth authenticator - something that is very important to higher education institutions in the US and the EU. Finally, the soon to be released Moodle 2.0 supports only PHP versions 5.2.8 and above.