I've used Delorme TopoQuads and TopoUSA, as well as Garmin's own Topo program. Garmin's is required for loading the topo maps into the GPS, but it isn't as useable as TopoUSA.
TopoQuads is a much more expensive option, but only if you need actual bitmaps of the USGS 7.5 minute topo maps. Interface is pretty much the same with TopoUSA.
