Free GM Resource: Fantastic Maps
Last November I linked to some Cartographer's Guild Tutorials. Many of those tutorials were done by Jonathan Roberts, who (I think) hails from Scotland.
His website, Fantastic Maps, is quite plain and at first glance looks like it might just be some quick advertising for his work. If you check out his tutorials and free maps sections you can find some great Free GM Resources.
Jonathan has taken many of those tutorials you might have looked at earlier and beefed them up quite a bit with some video. If you use Photoshop or GIMP just watching the videos can help increase your skill levels.
Of initial interest to me was a rather large collection of very cool free dungeon tiles. These tiles are quite a bit better than your average set of free (or I'd dare say paid) dungeon tiles. They are sized at 100 dpi for use with Virtual Table Tops and instead of giving completed rooms he has broken everything down into discrete elements that you can use to make whatever you need. These dungeon tile pieces would work well in Photoshop, GIMP, or Dundjinni.
While I highly recommend poking around the site and soaking in all you can, if you need a larger map for a special session you should check out Johnathan's maps on Drive Thru RPG. I've used them before and in my biased opinion they were some of the best "professional" maps I've ever used. Well worth the price.