What is the best scheduling / appointment software?

I am looking for a solution that would allow customers to book appointments and pay online. From what I can see, most solutions available are hosted and charge monthly vs. allowing you to install the software on your own server with a one time fee. Has anyone used anything in the past that you would recommend? For this particular site, the CMS is Drupal - so if it integrates nicely that is a huge plus.

Thanks in advance
Charlie