Really nicely done. I do not really see it in my workflow much because typing is faster than dragging around for me now and I am pretty familiar with grid concepts, but this will be very useful to explain the concepts to people. Very impressed with the dragging and snapping.<p>Suggestions: There are some bugs. Sometimes I cannot drag to create an offset, sometimes I can. Also, sometimes I can only resize the col from the right. Would be nice to support containers (fluid and non-fluid) and we can better visualize how it impact the behavior in relation to the width of the device. Also, I think you can easily support Zurb Foundation grid too.