Enter Easy Reflections, a PHP script that makes does just that. Available in PNG and JPEG/GIF flavors, it makes it wicked easy to generate reflected images. Just use an image tag like so:
<img src="reflect_v3.php?img=picture.jpg" />
There are extra parameters you can optionally pass, allowing you to alter the height and width of the resulting image, and adjust the size and tint of the reflection.
Don’t worry about putting too much stress on the server, there’s optional caching support.