I take a blank Google Documents Spreadsheet with re-sized cells:
The squares generally represent 10'x10' but my measurements get pretty liquid. Also I don't use my maps for tactical combats, just to give an idea of where everything is.
Here's the link to the blank map which you're welcome to copy and try my mapping system for yourself.
As a bonus, here's my folder of maps I've created over the course of my G+ campaign.