I found this info on IGN's board, and though I should post it here. Anyone have anymore?
1) Twin Rocks, if you follow the road that goes northwest out of Armadillo, you will run into it.
2) Pike's Basin, if you go west of MacFarlane's Ranch, you will run into Pike's Basin. Best to enter from Cholla Springs going east.
3) Tumbleweed, which is a ghost town in the Gaptooth area to the west. Lots of fun, especially if you approach it from the east rather than north or south. You'll go there eventually in a story mission.
4) Gaptooth Breach, west of both Tumbleweed and Benedict Point, best to enter from south.