Google Maps

Check out the beta of Google Maps. I particularly like that you can click and drag the whole map so you can follow a road without having to relocate it at each refresh. Excellent for figuring out where that railroad track next to my house ends up. Surprisingly, the search intelligence doesn't seem to be that great yet -- if you type in a street without a number it can't find the street.

Update April 5, 2005 - they recently added satellite images, too.