I registered thirdpartymodules.com and created a subdomain webmin.thirdpartymodules.com
Created a project hosted by sourceforge.net to host the website for the modules, and created a website utilizing php/mysql/perl to handle all the administration and hosting of these modules.
https://sourceforge.net/projects/webadminmodules
This site quickly became popular and was serving (at one point) over 670,000 hits a month.
Update: May 2007, Jamie Cameron (webmin developer) migrated all of this information into his main website.