Here's one keybinding I could not live without.
(global-set-key (kbd "M-j") (lambda () (interactive) (join-line -1)))
It joins the following line onto this one.
Let's say I want to collapse this paragraph-tag to one line:
<p class="example"> Some text over multiple lines. </p>
With point anywhere on the first line, I simply press M-j multiple times to pull the lines up.