I apologize to those that already know this sort of thing. I did a search and could not find anything like this. However that doesn't mean anything as it is probably up here somewhere...but just in case I was recently turned on to a very quick way to make simple cavern maps for RPG modules. Involves three layers in GIMP, PS, or whichever program you use.

First - get a grid. You can do this in GIMP or you can go download a suitable graph paper from here http://incompetech.com/graphpaper/plain/
And import it into GIMP. I use an 8 lines per inch grid from this site.

If you want to do it in GIMP start a new image. I normally use 1000 x 1000 pixels.

Go to Image -> configure grid. I use a small grid as I like to keep my maps small. Say 20 x 20 pixels. Select Solid for the line style
Then go to View -> Show grid to turn on your grid.

Go to Layer -> New Layer fill this layer solid black with the fill bucket.
Set the transparency on your black layer so that you can view the grid on the lower layer.
Then set the eraser tool to a 10 pixel or however large you like round brush. And simple erase you a new map.

Again add a third layer and use this layer to add text for room numbers and such.

using this you can whip out some real quick and dirty cave maps. It does not work so well with a square brush for more traditional dungeons.

Using this what you get is something like this. Here is a basic two cave level map that took all of maybe 10 minutes.

http://www.shadoespace.net/RPG/CaveMap.jpg

Again if this has been posted already(and I am sure it probably has somewhere) I apologize. NOT trying to take anything from anybody.