This site has been on my bookmarks for ages. This man is an utter genius for texturing, design and general all round geeky coolness. Although it's intended for 3d modelling, there is so much I've used here for mapmaking.

for example, take a look at his 'Dirty pitted metal' tutorial....pure gold. It's not metal, it's a landscape.....just change the colours!

If you like Science Fiction...well then you'll be right at home without having to adapt the tutorials to fantasy.