Writing Vim's Output to Files

Changing environment variables in vimrc

Refer to Vim Wiki's page on environment variables.
The tips uses paths in *nix in the examples. However, for Win*, some inexperienced users don't know whether slashes or backslashes should be used. In fact, backslashes should be used, and the string should be enclosed by a pair of single quote (I don't know if double quotes work.)
Then things should run. If you're unlucky and receive a runtime error in the next time you start Vim, then it may be due to the missing space characters around the equal sign.

Writing external commands to files

It's easy! (:r ![command] will do.)

Writing variables to files

Use the = register. First get into normal mode.

"=
[sth_to_eval]<CR>
p

The p have to be typed before any further changes of contents. Otherwise, it may not work.
For details, see Unix & Linux Stack Exchange question 8101.

Writing internal commands to files

A forum post has answered my doubt. I redirect interested readers to here.

Quick reference lists of some easy Vim commands

There's "100 Vim commands every programmers should know".
Another page: 130+ Essential Vim Commands