Why did you make it?
OK, so you have the What the SOT?! PCB ruler and can now keep your transistors straight, but now you've got to place some diodes. Sure the SOD names make a little bit of sense, but who can keep track of the difference between a DO-215AA and a DO-215BA? And where the heck do the MELFs fit in? Is a Micro-MELF more like an 0805 or an 0603? Instead of shaking your fists and screaming "FOR THE LOVE OF SOD!!!!!", grab this handy PCB ruler. Easily see at a glance alternate names for each package, and how big each footprint is in real life.
What makes it special?
Other PCB rulers have a few diode patterns with maybe one or two common names, but this has every single darned one I could dig up. It is the ultimate diode reference, every PCB designer or hobbyist who lays out circuit boards needs one!