The life of a DM is never easy. Work, work, work. Here are some free software tools which make my game-planning run much more smoothly.
1. Paint.net. This by far gets the most mileage - whether for creating maps or for random scribbling. For some reason when people recommend free art programmes on rpg forums they usually talk about GIMP, which I find fiddly and hard to use. Paint.net has a much shallower learning curve, does everything you want an art program to do, and is what the godawful Microsoft Paint always should have been.
2. Winshot, a free and incredibly easy-to-use screen capture device with which I take snapshots of bits and pieces of art to create flavour handouts and the like, and also create maps with via Google Earth.
3. Sakura editor, the absolute best text editor EVER. Unfortunately I believe that there isn't an English version, but if you download the program (click on the 'sourceforge project home' link) you should be able to get to an English download page. The interface is Japanese but it still writes English text and the keyboard shortcuts are all the same.
4. Google Earth. Perfect for searching for sections of the real world to turn into maps. Used to be one of my main methods of map creation. Here's an example:
5. Foxit Reader. I always hated Adobe Acrobat - it's slow, unwieldy, and dull - so Foxit takes care of all my pdf reading needs. It's incredibly fast and light in comparison to Adobe, with a much friendlier user interface.