We had a link to BoltDepot.com before which showcased some of their charts and reference materials. They have continued to expand their offerings.
These free printouts could be very handy if you’re trying to identify that unknown bolt or screw.
These ‘lay-over’ charts let you quickly identify fastener types and dimensions by simply matching up your fastener with the printout. The printable fastener tools series also includes general charts and tools for identifying and measuring fasteners, including a ruler and thread gauge.