I treated myself to a Nikon SB-600 flash for my D50. I'm very pleased with how it has improved my photos.
This photo was taken indoors (at the Ritchie's house in Cedar Hills, UT). In the past I've avoided taking photos indoors because of the shadows that the pop-up flash produces. But with an external flash, I'm able to tilt the flash head up and away from the subject, which produces a really nice, soft lighting.