Let me try doing that...

Webmin's been around for over 13 years, and is far and away the best GUI config tool for Linux servers. Possibly the only one worthy of the name. It's not hard to use - at all - if you can use a web browser and you have some clue of how what you're administering works (eg: it's no use trying to configure a mail server if you don't know a bit about what variables you need to plug in!), it's a breeze. As of 2007 (most recent stats I could easily find,) Webmin has racked up 8 million downloads with an extra 2 million added every year. It has modules for just about everything you might want to do with a server, including EC2 hosting, virtual machine management and more.

I've been administering linux systems for only slightly longer than Webmin's been around, and I don't know of another tool that even comes close. Why would anyone bother, when Webmin's so good and you can just make a module for it if it doesn't admin the thing you need it to? Of course, I'd love to be proven wrong too. :)