Digital cross processing
Tutorial, “‘Cross-Processing’ in Photoshop, from the Photoshop Fine Art Effects Cookbook: 62 Easy-to-Follow Recipes for Creating the Classic Styles of Great Artists and Photographers by John Beardsworth (O’Reilly Media 2006). [flickr discussion]
Tutorials 2.0 has an article on digital cross processing.
Cross-processing with the Gimp
The above Photoshop tutorials can also be applied to the Gimp, but there’s also Gimp-specific tutorials out there.
Becka Jayne’s step-by-step Gimp tutorial.
For further reference
Google search for “digital cross processing”