The best tool is the simplest.  It shouldn't get in the way of doing things, and it should be incredibly easy.  One of my favorite Agile dev tools is the digital camera.  Simply snap a quick photo of your whiteboard and you've got a good replacement for a $10,000 whiteboard with print / scan capabilities.  Plus, you then have it in digital format!  No need for scanning.

The problem is that you have flash reflections, etc, and that makes the image harder to read.  You need to do some image manipulation to really make it easy to read and understand.  And it should be in TIF or PDF format if you want to do OCR.  So, head to www.scanr.com  This great little web site takes a digital photo of a white board, and converts it to a very nice PDF document.  It eliminates most of the flash, rotates the image if you've taken a photo of the white board from the side, and saves it as a nice, high-quality PDF.  Check it out!