[offtopic]
ImageMagick is vastly more powerful than GD, each have their own strengths and weaknesses. To give a little idea of what can be achieved with ImageMagick visit Pixelmator which is an image editing application (for Mac, built using Cocoa) based on ImageMagick. It's not an online app, it sure is impressive (to me at least). :)
[/offtopic]

I did have a cool little page somewhere which allowed me to dynamically add/change various filters on an image using the imagefilter function -- I'll see if I can dig it up. :)