Adding my own, thanks @Mark for encouragement.

# [Xah Fly Keys](http://ergoemacs.org/misc/ergoemacs_vi_mode.html)

Designed from the ground up for emacs, with the goal of being the MOST efficient system, from years experience with ergoemacs-mode. Key choices are science based as much as possible, based on statistics of key frequency and key easy-to-press score. Most frequently used commands are mapped to the most easy-to-press keys. Other issues, such as grouping, and keybinding bigram, are also considered from 3 years of weekly experiment.

Interesting Points:

* All C-x commands are done by sequence of 2 to 3 single keys. In xah-fly-keys, C-x is never necessary. M- is never necessary neither.
* Does not conflict with any GNU emacs's keys, because it does not bind Ctrl or Meta (except C-7, C-8, but not essential). You can have xah-fly-keys on in insert mode, and use GNU Emacs the way you normally do.
* Also considered what set of commands results in max editing efficiency. So, the package uses ~80 custom editing commands. (for example, copy will copy current line if there's no selection. One single command to toggle letter case, instead of GNU Emacs's ~6 variations of upper/lower/region/no-region.)
* The implementation is as simple as possible. No macro, no advice, no complex remapping of keys, only a couple hooks are used. (good or bad?!)

Disadvantages:

* Like learning vi for the first time, you'll need one month to adopt.
* Not well known. Just released for public use this month.
* Currently support Dvorak layout only.
* No convenient key diagram for learning (yet).
* For major modes, you still need to use C-c. (so, you might use other packages such as god-mode or hydra to solve this problem.)

I'm the author, so be warned that I may be unconciously biased.
Feel free to comment or correct.