You may know about Finder's Get Info feature, especially if you're a Mac power user (or if you're just awesome). This handy little window'll give you the skinny on any item, quick as a wink. To invoke it, just select anything in the Finder and hit Command-I (or choose File > Get Info). When you do, a whole world of data will open up to you.
As you can see, you can do all sorts of things with this. You can find out the size of the item (which is especially handy if you're inspecting a folder or a volume) and its creation and modification dates. You could add a colored label to it or even change what application it opens with. If you scroll down to the bottom of that window, you'll get a preview of your item and a way to change the permissions applied to it as well.
I don't suggest you change those unless you know what you're doing, though. I mean, don't try to set your user Library folder to read-only, or anything. Please?
In any case, this is obviously an invaluable feature. If you want to open multiple Get Info windows, it's easy to do, too. Just continue selecting stuff and press Command-I for each one, or hold down Command, click on a few things to select them all, and then hit that same shortcut.
Here's where we get all crazy, though—you can check out what's called Summary Info on multiple items (useful, say, when you need to email a few files and you want to know how large they are together before you bother attaching them). To use this, hold down Command and click to select multiple items in the Finder, then press the keyboard shortcut Control-Command-I. If you prefer, you can instead hold down Control when Finder's File menu is open to make File > Get Info change to File > Get Summary Info.
Cool beans! Also, I think I should weed some stuff out of the folders that I did this on.
Lastly, it's important to know how to use Finder's Inspector window instead and how it differs from the first two choices. To invoke that gem, you'll use the keyboard shortcut Option-Command-I (or hold down Option when Finder's File menu is open to make File > Get Info change to File > Show Inspector).
Inspector windows are dynamic, meaning that as you move around your file system and click on different things, the window will update with the information for the newly selected items. So if you're trying to compare the file sizes of several different folders, for instance, you don't have to open a Get Info window for each one—just click between them to watch your Inspector window change. Heck, I think that's better than Christmas presents. Unless those Christmas presents involve chocolate or coffee. Mmmmmm.