There are three numbers in programming: 0, 1, and infinity.
This expresses the simple concept that, when considering some feature or capability, a program should not allow things of that type (0), it should allow a unique case (1), or it should process any arbitrary number of those things (infinity). It turns out to be a surprisingly good software design principle. Of course, there are practical physical limits imposed on computers, but the idea is what's important. (rambling wiki discussion)
Today, I just encountered a new saying that also seems to be a great truism:
There are three states for hard drives: in the box, almost full, and deceased.
As storage becomes cheaper, data seems to behave less like a liquid and more like a gas: it will expand to fill all available space. This is true of my hard drives, certainly. *sigh* Time for another trip to Fry's...