The Calibron Twelve Block Puzzle is an apparently simple bin-packing puzzle credited to Theodore Edison in 1933 and originally marketed by his company, Calibron. The goal is simply to arrange all the pieces so that they form a rectangle (hint: make a 56x56 unit square). However, difficulty of this puzzle is rated 5/5!
There have been a lot of variants of this puzzle; in fact, there's one on Thingiverse as I write this -- but don't look at that because it shows the solution. My Calibron 12 was designed from scratch to take advantage of what 3D printers can do. The raised dimensions on each "block" are accurate in units of 2mm (with allowance for printing tolerance), so the puzzle fits in a 3D-printed box that is less than 3"x2"x1" -- but the pieces are big enough to handle easily.
This was designed from scratch using OpenSCAD. However, the part dimensions are widely available. My primary reference for the dimensions was actually this code which claims to solve the puzzle. No, I'm not going to show you the solution... I haven't solved the puzzle myself (yet). This is also why I didn't pack the pieces optimally on the printer bed -- don't want any spoilers now, do we?