What I always wanted: commenting or uncommenting a line or region using emacs. Of course, there is comment-region, but this is much nicer:
(defun comment-or-uncomment-line-or-region ()
"Comments or uncomments the current line or region."
(comment-or-uncomment-region (region-beginning) (region-end))
(comment-or-uncomment-region (line-beginning-position) (line-end-position))
(define-key c-mode-base-map (kbd "C-/") 'comment-or-uncomment-line-or-region)
See also the corresponding Stackoverflow question.