What are the best open source web-hosting control panels out there? Are there any that are feature-competitive with commercial products like cpanel or plesk? What are your experiences working with them? I'm a command-line guy, so I've got a fair bit of sysadmin experience, but this is an area that I don't usually venture into. A client wants to manage more of their own server and resell hosting, so they are asking me to look into control panel software for them. They want to be able to quickly provision clients, and then allow the clients to manage their own email accounts, FTP files to their web server, etc.

I've some familiarity with cpanel and plesk, but would much prefer an open source version for budgetary reasons. I've seen the wikipedia page describing the various flavors of control panels that are out there, but would love to hear any first-hand accounts of installing, administering, maintaining, living with, or despising some of the open source packages that are floating around out there- particularly webmin and usermin.

The target server will be Ubuntu LTS, and most of the clients will be running wordpress blogs.