I found this article over at Grumbling Grognard, that describes how he made the map, using MS Office, for his take on Monte Cook’s Chordille Keep. GG started with Excel to create the squares, then drew the lines and squares. Then saved it as a bitmap and imported into PowerPoint to add numbers and color. GG says that he used a similar technique for his entry in the one-page dungeon contest.
I am sure one could use Open Office to do the same, Calc instead of Excel, Presenter instead of PowerPoint, or possibly Draw. One could also use GIMP with its layer capability to do variations with and without a grid, such as for the player’s map.
While there are specialty tools to make maps and dungeons, I find it interesting how one can use tools that one is already familiar and avoid the learning curve and expense of a new tool. (Here expense is not just monetary cost, but cost in time, disk space, etc.)