Simple Diagrams is a small-footprint Adobe Air application. It downloads and installs quickly and presents an intuitive, very clean interface to the first-time user. If you're already familiar with standard drawing toolbars such as those in Word or PowerPoint, you'll very quickly be clicking and dragging your way to your first picture.
I found the line tool is particularly handy. Instead of having to fuss with bezier curves (which I always find confusing), you just drag a central ball handle to change the curve of a line.
If you are wanting to provide presentation-quality diagrams, a simple white background should be a no-brainer.
For icons to jazz up your pictures, Simple Diagrams comes with a library of symbols, and there are additional premium libraries available at modest cost.