The Canon 50mm f/1.8 is by far my favortie lens. It's inexpensive, very sharp when compared with the entry level zooms and it's fast. I've been told Nikon's counterpart is just as much fun, but I can't see how since you have to use it with a Nikon. :p
Here are some shots with my 50mm lens.
50mm f/1.8 @ ISO6400 = campfire shots with no flash