Google Maps has got to be the hottest online map service with their new satellite imagery. I could waste hours playing with this feature. Just go to maps.google.com and type your address info into the search field (Yes, just one search field!). Now click the "satellite" button in the upper right hand corner of the map and, voila, an aerial photograph of your house. Click the "Hybrid" to get roads superimposed on top. And just click the mouse and drag to move the map.
For me Google Maps is just about as good as it gets. There are very few roads even out here in Northern Canada that don't show up. Their directions are also excellent, and there is even a search function that allows you to find various shops and services near your location. This alone puts them up over MapQuest and Yahoo Maps. But their international coverage is limited, so don't expect to use it to find your way around Budapest.
Click here to see a picture of my family's old house in Minnesota. I learned to canoe in the algae infested lake on the north boundary of our property.