Here is my emacs for C coding setup:

• Packages:
  • flymake – not using this right now as gcc won’t compile for the MCU I am targeting
  • compile
  • flyspell
  • auto-complete –
  • rainbow-delimiters-mode
• Formatting
  • M-x align – Align comments, equal sign, etc.
• Other tools
  • etags – M-. (jump to definition). See link for building TAGS table.
  • which-function-mode – List the current function in the mode line.
  • show-paren-mode – Highlight the matching paren or brace when cursor is next to a paren. The link also gives some other useful keyboard shortcuts.
  • org capture – Grab marked text into an org file. Alternately, try fixeme-mode. Also eproject extras may do this nicely.
  • semantic for Speedbar – Use a better C parser for the speedbar

My current config:

;;
;; C mode modifications
;;
(require 'compile)
(add-hook 'c-mode-hook (lambda ()
          (add-to-list 'compilation-error-regexp-alist 'x51)
          (add-to-list 'compilation-error-regexp-alist-alist 
                 '(x51 
                   "^... \\(\\w+\\) .*IN LINE \\([0-9]+\\) OF \\([^:]+\\):"
                   3 2 nil 1))           
          (flyspell-prog-mode)
          (rainbow-delimiters-mode)
          (setq compilation-auto-jump-to-first-error t)
          (which-function-mode)
          (show-paren-mode)
          (add-hook 'speedbar-load-hook (lambda () (require 'semantic/sb)))
    ))
;;Auto-Conmplete
(require 'auto-complete)
(add-to-list 'ac-dictionary-directories "~/.emacs.d/ac-dict")
(require 'auto-complete-config)
(ac-config-default)

Much of this is based on the useful notes in this SO post.