Harness the device's vibration motor using the vibrate() method, accessible on the navigator object.

Simply pass in the number of milliseconds you want the vibration to last.

navigator.vibrate(1000);

You can also specify an array of values. Every other value will be interpreted as a pause.

navigator.vibrate([1000, 500, 1000]);

For this example the device would vibrate for one second (1000 milliseconds), pause for half a second, and then vibrate for one more second.

Firefox led the way with browser support for this one, but Chrome and Opera have joined the party too.

W3C Specification - Browser Support

Matt West