mores

MetaBackGround will let you specify a post background by adding a custom field. This is not the sorcerer’s stone, but some users are not comfortable with php and editing of theme files, so all they need to do is install this plugin.

MetaBackGround

I once created this plugin for someone ... they needed to set backgrounds on a per-post basis.

Gecka

PHP 5 required. Please be sure to check your PHP version before marking the plugin as broken.
BgStreacher plugin for WordPress implements the great bgStretcher Jquery script to your wordpress blog.
It allows you to add a large image (or a set of images) to the background of your web page and will proportionally resize the image(s) to fill the entire window area. The plug-in will work as a slideshow if multiple images mode is used (the speed and duration for the slideshow is configurable)..
For example, google uses such a hack for their background image feature
The plugin supports all the bgStreacher features, like slideshow.

Gecka BgStretcher

The plugin allows you to add a large image (or a set of images) to the background of your web page and will proportionally resize the image(s) to fill …

MetaBackGround vs Gecka BgStretcher