If, like me, you are always writing little pieces of Elisp to do some task or another or if you have a lot of ELPA packages installed, chances are it's hard to find key sequences for them. In theory, 【Ctrl+c】 is reserved for just this purpose
but in practice rather than being reserved for users it gets used by almost every package. Even my beloved Org mode is guilty of this. 1 On the other hand, what choice do they have? There just aren't enough key sequences based on the 【Ctrl】 and 【Meta】 keys.
Emacs, of course, can also make use of the 【Super】 and 【Hyper】 keys but modern keyboards no longer have these keys. All is not lost, though, because you can map some otherwise unused key to 【Hyper】 or 【Super】. Xah Lee has a useful post on defining hyper and super keys for Windows, OS X, and Linux. One trick that he misses for OS X is using the 【fn】 key for this. That's something I learned from Magnar Sveen. I wrote about that here (don't worry, you can still control music and the other special functions).
I have mapped several key sequence that begin with 【Hyper】. You almost never trip over a predefined sequence and even if you do that command invariably has some other key sequence assigned to it too. Take a look at Lee's post and experience the joy of a whole new key sequence space.
Update: Corrected my misinterpretation of the reserved key rules. See the comments for further information.