The Imagene gradient generator Drupal module generates 1D and 2D gradient images based on [Bézier control points](https://en.wikipedia.org/wiki/B%C3%A9zier_curve) encoded in the file url.

animelion on drupal.org asked me for some example images Here are some!

You need javascript enabled to see the gradients.
(imagene does not need javascript, but this blog post does)
Use your favourite web developer tools (Firebug etc) to inspect the elements and see the background image url.

Vertical gradients

With only two colors, you can achieve a lot of different gradients: Linear, cubic, etc.

Transparency gradients