Other
Trigonometric Identities
I always forget these simple trig identities. So here are a couple of links to pages that have them. The content is basically the same, I'm linking more than one just in case some of them go away.
http://www.clarku.edu/~djoyce/trig/identities.html
http://www.sosmath.com/trig/Trig5/trig5/trig5.html
This page has reference sheets on many different topics. I don't know what the other ones are like, but the trig reference sheet is nice.
EDIF Mode for emacs
Not many people like to look at EDIF (Electronic Design Interchange Format) files, but it comes in handy in the work I do with FPGAs. VIM actually comes with an EDIF mode which includes syntax highlighting, but I'm more of an emacs guy. To get syntax highlighting for EDIF files, download my edif-mode.el file and place it in your ~/lisp directory, then add the following content to your .emacs file:
;; Emacs Mode
(setq load-path (cons "~/lisp" load-path))
;; Load EDIF mode only when needed
(autoload 'edif-mode "edif-mode" "EDIF mode" t )
;; Any files that end in .edf or .edn should be in EDIF mode
(setq auto-mode-alist (cons '("\\.edf\\'" . edif-mode) auto-mode-alist))
(setq auto-mode-alist (cons '("\\.edn\\'" . edif-mode) auto-mode-alist))
;; Any files in EDIF mode should have their keywords colorized
(add-hook 'edif-mode-hook '(lambda () (font-lock-mode 1)))
This should auto-load the EDIF mode when opening any *.edn or *.edf file. This doesn't help you write EDIF by hand (as if anybody would ever want to to that!), just look at it. It doesn't do any smart indenting or anything, just gives pretty colors to the keywords. Still, I've found it pretty useful, and easier than learning VI.