Just about any pre-constructed maps are worth the cost to the DM, in my humble opinion. These are excellent for those times when the players decide to explore something you mentioned out of hand. Pair this up with dicegeeks Random Tables and you have an excellent set of resources for keeping the play smooth and uninterrupted.

This is a concise assembly of many tables which help me run a game. One may purchase all separately, but having them in a single file is a time-saver for me.