I've written a php_block that grabs any file or internet source, caches it for a specified amount of time (or on a daily basis) and then displays that cache in a php block. I use it to display a random quote every 15 minutes, and a daily image chosen by random from my iPhoto galleries.

The block code doesn't do any of the choosing, all it does is download the page, cache it, display it, and expire it as necessary. You can see it at Cerebus the Gopher.