phpThumbs is a great script for resizing images on the fly. The script lets you create thumbnails or other variations of an original image simply by calling a specially crafted URL.
<img src="../phpThumb.php?src=images/disk.jpg&w=200" alt="">
That line of code will produce a resized version of the original at a width of 200 pixels. Easy as. The only drawback, however, is that phpThumbs can be slow, even with caching.