Hi, Will--and welcome. Make sure to post an intro in the "members introduction" threads so we can formally shake your hand and slap you on the back as you walk in the door!
I think if you know GIMP/Inkscape you need look no further for a cartography app. Check out the tutorials in the "tutorials" threads and you'll see the amazing output that program can achieve (especially see RobA's new GIMP tutorial!!!). The combo of Inkscape for vector creation/layout, then GIMP for finishing is also extremely useful. And, of course, all open source!!!!