The Dungeon Master's Toolkit
NPC Generator by Jamis Buck. The classic!
Town Generator by Jamis Buck.
Dungeon Generator by Jamis Buck.
Treasure Generator by Jamis Buck.
Dragon Generator by Charles Greathouse. This is a fantastic tool.
Domesday Book This is cool. It's like the DM Medieval Kingdom Lego Set. Calculate population densities based on temperature, tells you how many people are in the town and what they do for a living.
Squid.org Tablesmith-type generation of names, quests, and other stuff. If you don't download tablesmith, use this resource. It is one of my personal favorites.
Autorealm This is a mapping program, and it is the best I have used. It is both flexible and easy to use.
Izandawo This is the OTHER best free mapping program, and it is also the best I have used. If you're going to choose one free mapping program, I think it's probably this one, not Autorealm.
Tablesmith Combine tables to generate results. That description sounds lame, but I assure you that this is a fantastic tool. It contains name generators, herb generators, and much more that you can use right away. Don't miss this.
A collection of tile links (print and use as pre-illustrated battlemats) is in Miscellaneous Resources