Surround.vim

It's easiest to explain with examples. Press cs"' inside

"Hello world!"
to change it to

'Hello world!'
Now press cs'<q> to change it to

<q>Hello world!</q>
To go full circle, press cst" to get

"Hello world!"
To remove the delimiters entirely, press ds".

Hello world!
Now with the cursor on "Hello", press ysiw] (iw is a text object).

[Hello] world!
Let's make that braces and add some space (use } instead of { for no space): cs]{

{ Hello } world!
Now wrap the entire line in parentheses with yssb or yss).

({ Hello } world!)
Revert to the original text: ds{ds)

Hello world!
Emphasize hello: ysiw<em>

<em>Hello</em> world!
Finally, let's try out visual mode. Press a capital V (for linewise visual mode) followed by S<p class="important">.

<p class="important">
  <em>Hello</em> world!
</p>