The Astronomy Picture of the Day, created and maintained by Goddard Space Flight Center, NASA, and the Michigan Tech. U., is a fantastic way to get a glimpse into the latest images of the universe.
The RSS feed that these pictures come out of leaves a little to be desired, though.
I've created this mirror that provides a simpler, properly encoded feed for their content. All of the content remains at NASA, and I have no association with any of these organizations. Just a fan.