This is as special interest as any software can be!
At least in Germany, food containers by LocknLock are everywhere. We have a TV station that does nothing but sell LocknLock containers over the phone! Well, they sell other stuff too, but I guess LocknLock single-handedly carries that whole ordeal. Anyway.
I like their food containers, I really do! But I also like stuff that stacks well and while LocknLock in fact does stack well they have about a gazillion different containers and not all sizes are compatible and going through all the measurements and figuring out what containers are multiples of what other containers was getting annoying.
So here it is: LocknStack! Pick a container and it tells you what other containers fit above (or, in fact, below, but you'd have to be some special kind of monster to put the bigger container on top) and also what containers fit neatly. Currently only the HPL line of products is supported and that was copied to a machine readable format by hand like the luddite I am.