This thread is meant as a sort of annotated compendium of helpful resources, software and tutorials that can enable you to create your own maps for your campaigns and/or campaign worlds. Though there are some really good commercial resources on map-making out there, I've focused here on resources that are all free or freeware.
h3. Helpful Resources for Map-Making:
1. "A Magical Society: Guide to Mapping":http://paizo.com/store/downloads/free/v5748btpy82tc, is a free 37 page pdf that covers everything from the axial spin of the planet, to its climate and ecology, to the cultures that inhabit it. This is an extraordinarily useful little manual, especially given that it is absolutely free!
2. "Medieval Demographics Made Easy":http://www.io.com/~sjohn/demog.htm is a short online, researched article aimed at explaining to the average DM how things such as population density and spread, agriculture, and economic goods and services function within a Medieval setting.
3. "The Doomsday Book":http://www.rpglibrary.org/utils/meddemog/ is a supplemental aid to the article on Medieval Demographics. This site offers a sort of "Settlements Calculator"; you plug in inputs such as the climate and geographical spread of one of your civilizations, and the calculator generates outputs such as a break down of population (and its spread), specific details relating to the villages and settlements of your civilization, number of castles and fortifications, and even a breakdown of what kinds of goods and services are likely available in those villages (including how many butcher, bakers, thatchers etc. the village would likely support).
4. "The Cartographers Guild":http://forum.cartographersguild.com/index.php?referrerid=15127, a site dedicated to supporting those who enjoy making maps, whether you are a complete novice, an amateur, or a professional RPG cartographer (and there are several of these within the Guild). Not only are there great tutorials for map making, but there are also very instructive discussions on related topics that bear on our conversation here.
h3. Free Software for Map-Making:
1. "GIMP":http://www.gimp.org/ is a raster based Image Manipulation Program similar to Photoshop. It is a freely distributed piece of software for such tasks as photo retouching, image composition and image authoring.
2. "Inkscape":http://inkscape.org/ is an Open Source vector graphics editor, with capabilities similar to Illustrator, CorelDraw or Xara X.
3. "Wilbur":http://www.ridgenet.net/~jslayton/wilbur.html is a freely distributed bit of software that works with height fields (it can be used to simulate somewhat realistic erosion on maps)
_NOTE: most of these tutorials are all hosted at the Cartographers Guild; you will need to create an account there to access them._
h3. Tutorials On Making Regional or World Maps:
1. "How to Create an Artistic Regional Map in GIMP":http://www.cartographersguild.com/showthread.php?1142-Award-Winner-Using-GIMP-to-Create-an-Artistic-Regional-RPG-Map this is a good _Beginner_ level map tutorial in GIMP (written by RobA). The style of map is somewhat realistic.
2. "Antique Style Maps in GIMP":http://www.cartographersguild.com/showthread.php?15011-Antique-Style-Maps-in-Gimp. This is another good _Beginner_ level tutorial in GIMP (originally written for Photoshop by Ascension but translated to GIMP by Candacis). The style of the map, as the name implies, is more traditional.
3. "Atlas Style Map In GIMP":http://www.cartographersguild.com/showthread.php?4084-Award-Winner-My-atlas-style-in-PS/page27 (located at the bottom of the page). This is another good _Beginner_ and _Intermediate_ level tutorial in GIMP (originally written by Ascension for Photoshop, but translated to GIMP by Gidde). The tutorial will give you an attractive pseudo-realistic atlas style map.
4. "Hand Drawn Mapping for the Artistically Challenged":http://www.cartographersguild.com/showthread.php?10655-Hand-Drawn-Mapping-%28for-the-Artistically-Challenged%29 this is a great _Beginner_ level tutorial for a more hand-drawn style of map (written by Gidde).
5. "Photorealistic Maps in GIMP & Willbur":http://www.cartographersguild.com/showthread.php?18280-Eriond-A-Tutorial-for-GIMP-amp-Wilbur this is a slightly more _Intermediate_ level tutorial that walks you through how to create a map in GIMP and add realistic looking terrain to it in Wilbur (written by myself).