That's a great idea! In fact we're actively doing something similar that at http://www.roborealm.com/ for free!
The free or open source tools each have their benefits. OpenCV is certainly the best known and probably widest scope but typically requires c++ programming to make use of it. Others like ImageJ (Java based) have a more microscopy focus and nicer GUI .. there are many out there as both libraries and/or applications. For a comprehensive list that we've found check out http://www.roborealm.com/links/vision_software.php
Enjoy!
STeven.