After my past reviews of some of the Popplock series (T2, T3, T4) and the Danlock it may look like collecting puzzle locks is only for puzzlers with a decent amount of spare cash lying around. However I'd like to put forward this puzzle as an example that not only succeeds as a decent puzzle lock but also won't financially cripple anyone who buys one.
This is called the Lunatic Lock, and it was designed by Gary Foshee. The body of the lock is made from aluminium, which has a brushed finish and has the puzzle name engraved into the side and the shackle and bolt are made from stainless steel. I'll mention now; the quality and finish of the metals are not amazing (some of the edges are pretty sharp) but keep in mind that it only costs around $15 as opposed to over $200 as some of it's counterparts.
Holding the lock in your hands you will notice that there is a fair amount of movement between the shackle and the body, and in my opinion this could've been tightened up a bit, but apart from that the only other movement you will notice is that the bolt going straight down into the lock will move out from the body until it makes contact with the shackle which is seemingly holding it in.
The shackle will rattle around but it won't move more than half a millimetre or so in any direction, and the bolt will seem to be able to move freely one minute and then be stuck in place the next.....very curious.
The goal is obvious; open and close the lock, and I managed to do this in around 20 minutes or so. The mechanism is simple, so simple that it actually makes it more difficult for accomplished puzzlers to solve it as they tend to over-think what's going on inside. As a puzzle concept it is very clever, I believe that if the measurements and overall quality were tightened up a bit then the puzzle would not only look nicer but it would also become more difficult as a result.
The Lunatic Lock is well worth the pretty meagre price that it costs, and I seriously recommend adding one to your collection. I plan at some point to make a version for myself out of either wood or plastic with a tighter fit to see how it works out, but I'm sure I'll update about that when I get around to it.
Also check out Brian's opinion of this puzzle.