CSS3 PIE (Progressive Internet Explorer) is an IE attached behavior which, when applied to an element, allows IE to recognize and display a number of CSS3 properties.
PIE currently adds full or partial support to IE 6 through 8 for the following CSS3 features:
multiple background images
linear-gradient as background image
And in IE9 PIE adds support for border-image and linear-gradient.
CSS3 PIE Homepage