A few months ago, James Padolsey introduced a cool greyscale technique for non-IE browsers. His technique inspired SohTanaka to come up with a workaround with a similar effect.

Greyscale Hover Effect with CSS & jQuery relies on CSS Sprites and a few lines of jQuery, but requires a bit of preparation before it can be implemented. It is not recommended for large scale projects and probably best for displaying portfolio pieces.

Requirements: jQuery Framework
Demo: http://www.sohtanaka.com/web-design/examples/hover-over-trick/
License: License Free