Topos online
The website www.backcountrymaps.com is a really great site for looking up topos. I have checked out quite a few places online for topo maps, and none is as smooth and easy to move around in once you have located your desired area. Unfortunately there are rough edges since the site is a project being run by some folks in their spare time. It seems there is no way for ordinary layman to register, and if you are not registered then finding that area that you are interested in the first place can be a little challenging. You can move around on the map till you find what you like though.
Another neat topo thing that I got working the other day is topos in google earth. Here you can download a file that you can then use with google earth to view topos of anywhere in the US. The only negative for this is that it is a little slow loading the various maps so moving around can be just a little tedius.
Although neither of the mentioned topo sources are of a high enough quality that you wouldn’t want to buy a map before going out in the backcountry, I think both will be very useful in planning future trips.