I did one a while ago. I was dissatisfied with the ones I saw so I coded my own. It's basically a rectangle with text in it that gets positioned inside a cropping container (CSS, overflow:hidden), and gets a nudge downward every 100 milliseconds. Actually the scrolling text part is a stack of smaller rectangles positioned absolutely inside a container with relative positioning. When one rectangle scrolls out of the frame (ie its "top" is greater than the container's "bottom") I reposition it back on top of the stack and the whole thing keeps looping and scrolling.
Look for something built with Mootools or JQuery; then you can be more confident that it'll work in all browsers.