Aaron Hertzmann writes: "My collaborators and I will present a paper called Image Analogies at SIGGRAPH 2001 this summer, where we describe a machine learning method for 'learning' image filters for example. For example, given a Van Gogh painting, the algorithm can process other images to look somewhat as if they were painted by Van Gogh."
"It can also 'texturize' images based on a sample textured image, e.g. to create landscape photos. It can do many other types of filters, as long as you give appropriate 'before' and 'after' examples to learn from." I especially like the idea of inferring a high-resolution image from a low-res one. The software is available for Windows and Unix, and "the source code is freely distributed for educational, research and non-profit purposes."