Here are some pretty reference cards I made a while back:
- Scale Landmarks: What's something you're familiar with that is about 10 nm big? How do the speed of continental drift, a raindrop, a champion sprinter and an SR-71A Blackbird compare? What is the range between the least massive (electron) and most massive (universe) objects science can describe?
- Periodic Table
- Periodic Table, Flat -- material properties as a table and not as Mendeleev puts it.
- Mechanical, Geometric and Material properties of Screws, Bolts and Fasteners - probably the most useful among these, this gives thread geometry, decimal inch/screw/metric equivalents, mechanical strengths, torque ratings and more. Super handy for machining or general shop use.
- A similar table for AN Hardware (milspec fasteners used in airplanes, racecars and hot rods).
- A flat table of decimal equivalents: decimal and fractional inch, metric, and standard (US) screw sizes.
- ASCII Chart - easily index up hex, octal, ascii, symbol font/latin font/DOS font values for characters.