The OCSP Responder of CAcert is available at http://ocsp.cacert.org/ and http://ocsp.cacert.org:2560/ To activate it, tell your client to use OCSP (take a look at the settings). All the certificates that were issued after 16.5.2005 should have the OCSP Service URL automatically included, and should work with OCSP, when you activate it in your client.
The OCSP Responder runs at the port 2560, the following Apache rule makes it available on Port 80:
RewriteCond %{CONTENT_TYPE} !^application/oscp-request$ RewriteRule ^/(.*) http://localhost:2560/ [P]
The hostname ocsp.cacert.org is distributed with round-robin DNS. If you would like to host another OCSP responder for CAcert, pleaseg contact us.