SToyan Stefanov has posted some examples of what sort of output can be made with the imagefilter function in PHP.

Here's the list of filters he applies:

• Brightness
• Grayscale
• Contrast
• Colorize
• Edge Detect
• Emboss
• Gaussian Blur
• Selective Blur
• Mean Removal
• Smooth
• Negate

And, as a sort of a bonus, he includes a lazy way to make your image look like it's sepia toned.