Anystretch is a jQuery plugin that allows you to add a dynamically-resized background image to any page or block level element. The image will stretch to fit the page/element, and will automatically resize as the window size changes.
Handy when having a fluid layout/responsive design.
UPDATE: Reader Oemebamo pointed out that this can be achieved with CSS, using
background-size: cover;. Turns out
background-size is widely supported (thought it was Chrome only).