This module allows you to enter ImageMagick command line options as an image effect in a Drupal 7 image style.
Note: This module has been renamed following the incorporation of Imagecache into Drupal 7, since there are no longer any imagecache actions and only image effects.
Drupal 7.x: Imagemagick Raw Effect
http://drupal.org/project/imagemagick 7.x-1.x-dev or 7.x-1.0-alpha1 is required.
Drupal 6.x: Imagemagick Raw Action
Works with the releases of ImageAPI and ImageCache for Drupal 6.x.
Take a look at http://www.imagemagick.org/Usage/ to get an idea of the sort of things you can do. But keep in mind that the Drupal image effects api puts constraints on what is possible, not everything that imagemagick can do is possible in the context of an image effect.
There's also an article http://perlucida.com/blog/features/css-sprites-using-imagecache-and-imag... discussing one of my uses.