For all of y'all coming here via the Corner, I don't usually make a big deal of it, but I work for an IT company that does a lot of GIS work, which means I get to play with a lot of mapping applications and the like. I'm not so much a map geek as an assistant to mapping professionals of a sort. However, like a lot of wargaming enthusiasts, I do have that giddy fascination for a good contoured map, the representation of reality in two-dimensional color and false-perspective.
DeLorme's Topo USA 4.0 is installed on my work machine, and to be strictly honest, it's my primary tool. We've got various real GIS applications all over the place, and I do a lot of hand-editing on detailed GPS-indexed agricultural data, but that isn't the exciting stuff, is it? Topo USA 4.0 gives the pretty, pretty pictures, and it's actually a decent resource for finding locations in the continental US. Most often, I use Mapquest if Topo USA is letting me down, but in most particulars the software beats the website, especially if you've turned on the topographic features on Topo USA. For online topographic neatness, you can always go with Topozone, but that website is essentially a really comprehensive set of digitized topographic maps at various resolutions, rather than a truly interactive map webapp. Outside of the continental US, you'll be wanting Multimap, which I've found to be pretty useful for Canadian informational searches, and for following along with American blitzkrieg warfare in the Middle East.
As with most things, Google is your first stop for finding the unknown, or refreshing the half-remembered.
Real GIS professionals play with ArcView and other, better tools, I'm told. I used to use ArcView a bit, but frankly, I'm not a real GIS professional, and it's a monstrous pain in the ass. We have our in-house GIS applications which I can't show you due to it all being only for paying customers, sorry. You can contact us if you're really interested, but I doubt we're going to get many agriculture professionals looking for GIS work around these parts, no?