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Hubble took the deepest visible light image yet made.
In 2003, an astronomer (and friend with whom I worked on a Hubble project) named Tom Brown pointed Hubble at the outer fringes of the Andromeda Galaxy, a nearby large spiral like our own Milky Way. Using the Advanced Camera for Surveys, he commanded the space telescope to basically sit and stare at one spot for a total of three and a half days. His goal to was to be able to get good data on very faint stars in Andromeda, to characterize the way stars form in the galaxy.
He certainly was able to do that (and found many stars younger than expected; in Andromeda's halo the stars were several billion years younger than in our own halo), but what he also got was the deepest optical image of the Universe ever taken. Stars down to 31st magnitude can be seen in the data -- those are stars one ten-billionth as bright as what you can see with your unaided eye!
The image here shows different regions in that deep image. You can see faint background galaxies, stars in both Andromeda and the Milky Way, a densely-packed globular cluster, and much more. If you dare, download a monster-sized version of the whole schmeer to see how powerful a space telescope can be.
