This is what I use:

http://earth.google.com/ (Free)
Gives decent quality but most ofthe pix are during summer and terrain features are harder to make out, though the tilt feature is helpful to pick out large elevation changes.


http://www.terraserver-usa.com/(Free)
Provides ok quality but gives you urban area photos, aerial photos, and topo maps.

http://worldwind.arc.nasa.gov/(Free)
I just became aware of this one and it seems to provide much better quality aerial photos taken during times of low foliage so you can make out trails and even individual trees. But it runs slow on my machine.

http://www.maptech.com/index.cfm(Not Free)
A good reasonable package is Terrain Navagator from Maptech. Good quality topos for $99.95 for your entire state, or several states if you live in the New England or Mid Atlantic regions.

Some options to consider.

Bob