Managing Multiple WordPress Sites: An Open Discussion & Demonstration

If you're a freelancer or run an agency, you've more than likely been in a situation where you have to manage multiple installations of WordPress. How do you handle your backups?
How do you manage your plugin & theme updates?
How do you monitor if your websites are working?
How do you turn off a site if the client stops paying?

We'll be doing an open forum guided discussion because everyone has good ideas when it comes to managing multiple websites. If you have a tool (or tools) you prefer, please come prepared to discuss them.