After downloading so many awesome backgrounds here, I decided I needed to switch between them hourly. I found a shell script to do it on Michel's page but it gave me errors occasionally and only scanned one directory, so i wrote a new one in Python. I figured someone else might find it useful, so here it is.
(To make it run hourly, run "crontab -e" and put "01 * * * * python /path/to/ranwp.py" on a new line in it, then save)

http://divmod.org/users/washort/ranwp.py