You're driving on the interstate and pass an 18-wheeler with a small sign in the shape of a diamond. There's a number on that sign. What does it mean? There's no way to know!
Wrong! You can find out what it means. That number is code for some sort of chemical and I have
a database chock full of these numbers. Numbers are between 0004 and 9269 with most
being below 3356. There are two systems, UN being the more popular one. Search away.
[Data is from circa 2000.]
But really, Wikipedia probably has a much better, up-to-date list. I first made this database before Wikipedia even existed. It's now quite deprecated.
Office of Hazardous Materials Safety
Wikipedia: List of UN numbers