Normal mode

^W + ^= Make split windows equal size.
@: Repeat the last Ex command.
gv Select last visual select.
= Auto-indent.
=G Auto-indent to the end of the file.
=i} Auto-indent inside de bracket.
s Deletes the character and enters Insert mode.
f Searches for character in line. ; searches for next instance, , searches for previous instance.
* Searches for all occurrences of word that's under the cursor.
daw Delete a word. Different from dw because it deletes the full word, even though the cursor is not placed on the first character.

Insert mode

<C-h> Delete back one character (backspace).
<C-w> Delete back one word.
<C-u> Delete back to start of line.
<C-k> Delete forward to end of line.
<C-r> Pasted yanked text without leaving the insert mode. If yanked text contains new line characters, <C-r><C-p>0 will take care of fixing indentation issues.

We can visually select all 3 lines and then run command :normal A; that will execute A; (append ;) for each line. Alternatively, we can run same on the whole content of the file with :%normal A;. What if we did a single modification on the first line of the above snippet appending ; at the end of the line with A;. We can repeat that command by selecting the lines 2-3 and running the dot command over visual selection with :normal .
:sort Sorts selected lines alphabatically.

%s/foo/bar/g Replaces foo by bar in whole file.
:args app/_/scss/modules/*.scss Loads all files in modules in args.
:argsdo %s/replace/with/g Replace something in all files in the arguments list.
:argdo update Save all the files in the arguments list.