In short, a fully printable, fully working padlock with a detachable shackle.
And no, this is not a joke lock, this has an actual working five-lever tumbler mechanism and only opens with the correct key (or by lockpicking).
See how it works: http://youtu.be/yzBI0HB2o-I
Use fine settings to print the body, bolt, key and tumblers. I used 0.20 layers and 0.36 track width.
Update 30.7.2012: There's a file lever_padlock_tumblers.zip which contains the tumblers as individual STLs. It may be more convenient to get and print the tumblers as individual objects (using Slic3r's plater, for example) instead of re-exporting them as a batch every time when changing the bitting. The included key works with tumblers 3, 7, 4, 0, 2 counted from the bottom up.
Sand the top surfaces of the tumblers and the bolt, these must slide against each other freely. Sand the bolt hole in the body, the bolt pin, the main shaft, the key guide shaft and the round holes in the tumblers and in the end of the key, if needed. (Update: I have only needed to sand the bolt and tumbler top surfaces and the key bitting, even for a 10-tumbler lock.)
Assemble the bolt into the body first and check that the key operates it correctly without tumblers. The end of the key that operates the bolt may need a little sanding (it should be rounded).
Assemble the tumblers into the body (the one at lowest Y position goes in first), checking that they move freely.
Then finish the key. As printed, the tumblers probably won't fit between the high bitting positions, so widen the low positions by sanding until the tumblers slide into those freely. Try to operate the lock with the key - it should now work, but if it doesn't, check what's wrong and finetune things with sandpaper if necessary.
Print, with coarser settings, the shackle and one of the chassis pieces (one is secure and one comes with a window to see the mechanism in action). The mechanism body can then be inserted into the chassis and the lock should be fully operational.
If you want to change the bitting, just modify the bitting value array in the scad file and export the new tumblers and key.
Everything is parametric. Some things (like chassis wall thicknesses and tumbler count) can be changed easily but the mechanism details have a lot of interdependencies, which need hand-tuning if something is changed. The mechanism itself is already smallest I could make reliably work with reasonable effort.