You can use Inkscape as discussed in How Do I Make Complex Drawings I Can Edit in Windows and Linux? or if you have Microsoft Office you can actually use Excel.
Using Excel for drawing seems strange at first. However, if you change the column width to "2" you'll get a grid. Under the format menu, you can select snap to grid. In this mode, it's possible to draw objects that have consistent sizes and connect them with lines that move with the shape. Most of this time, this is all you'll need for quick drawings.
The only drawbacks are:
- resizing a row or column will resize your shapes,
- it's not really possible to select objects using a "lasso" method.
Regardless, using Excel this way has a lot of advantages.