Plesk is a hosting automation solution that gives hosting providers full control over the managed Web hosting servers and simplifies setup and management of user accounts, web sites, and e-mail accounts.
Plesk is stable, secure and reliable, but what you will like about it the most is that it will save you and your technical staff much time and effort: all your customers (and customers of your resellers) will have personal isolated control panel environments and will be able to manage their sites and e-mail accounts on their own — without bothering you.