free-keys 
Show free bindings in current buffer. Based on https://gist.github.com/bjorne/3796607
To use, call the command free-keys. This package takes into account the major mode bindings as well as any bindings occupied by minor modes active in current buffer. If called with prefix argument C-u, you can specify a prefix map to be used, such as C-c or C-c C-x (these are specified as a string).
You can customize the variable free-keys-modifiers if you use non-standard modifiers, such as H for hyper, s for super or S for shift. By default this list contains C, M, C-M and no modifier.
You can customize the variable free-keys-keys if you use non-English keyboard layout and want to show free bindings for keys such as Δ, Γ, ΓΆ, Γ¨, Γ₯ etc.
These bindings are available inside the *Free keys* buffer:
| Key-binding | Description |
|---|---|
b |
Change the "active" buffer |
p |
Change the prefix |
q |
Quit |
Installation
The easiest way is to install this via package.el from MELPA repository. If you want to install manually, clone the git repo and add it to your load-path:
(add-to-list 'load-path "path-to-this-git-repo")
See also
The bind-key package by @jwiegley does the "reverse thing", that is, it shows you what bindings you've already used and if you've shadowed some built-in bindings. I highly recommend using it in tandem with this package.