OS X has such a multiplicity of features (and new ones are added with every update) that no one can be aware of them all. Within this vast array I often come across one that is new to me and very useful.
This particular hidden gem was, as far as I know, introduced in Lion. It is a really handy overview of the amount of storage available on all of your hard drives, internal and external.
You get to it by clicking on Apple Menu/About this Mac/More Info… Then click on Storage.
There are also some other choices, Displays, Memory, Support and Service, that you might want to explore.