I use
www.topozone.com for topographical maps, and both
www.mapquest.com and
www.googlemaps.comusing the aerial views as well.
Google Earth has some really neat features including a ruler and path feature that let you measure distances in feet, yards or miles. You can download it for free here -
http://earth.google.com/download-earth.html
The aerial maps and even google earth don't do a really good job of giving you a true perspective of the actual rise and fall that a topographical map will. On my property, google earth gives me an elevation change of about 20 feet when in fact there is about an 80 foot elevation difference. A topographical map will really let you see the funnels and drop offs well.
FWIW, Nathan