Is there a device that will allow a bathroom fan to run for 5 minutes after it is switched off?
You want a timer switch. They are quite common. As Tester101 states, you have to make sure the switch is either:

a. rated for the load of the motor (most should be for a standard fan) or, if not...

b. you need to have the switch control a relay, which in turn will turn the fan off/on. I did this to install the lutron timers that I like on a ceiling heater unit (which was more of a load than the fan):