NASA's Wide Field Infrared Explorer: look at the slide show for fantastic pictures of different types of galaxies.
The Andromeda galaxy
Amazing Space "Hubble Deep Field Academy" lets students take on the role of scientists and use HST photographs to estimate numbers of stars, classify galaxies, and estimate distances.
Space Telescope Science Institute: images taken by the Hubble Space Telescope, organized by topic. Click on the links for information about each picture.