Well, as i've discussed here before, I'm a Geomatics Applications specialist (or will soon be.. i'm in school)

we're doing a remote sensing course (and let me just say.. holy crap)
our teacher, who's worked for the UN and other places has graciously informed us of many open source GIS solutions out there (and i thought how perfect it would be for doing our type of fantasy mapping... without ripping off or spending thousands on ESRI's info, etc)

i'll see how many i can get up here.. but this one really interested me...
he said its fairly terrible for vector mapping, but for raster (images) it has almost the same, if not more functionality as ArcGIS...

its called ILWIS, and while i'm in class right now and can't look it up, i'll be sure to do so when i get home, and post an update... apparently it does hydrography, dems, as well as many imaging aspects of GIS...

heres the link

more to come (hopefully)
he's got a crapload of places to get free information from world (for all you modern cartographers out there... work well for post apocs and such too i'd imagine)