Went hunting for topo maps again recently, and found an interface called TopoView which is so much better than the USGS store and anything else I've seen that it's dumbfounding:

http://ngmdb.usgs.gov/maps/TopoView/viewer/

It has historical & regional maps as well as the topographical maps. It's easy to browse. It has built-in support for pinning maps on Pinterest. Really.

Huge props to whatever unsung team at USGS pulled this together.