Martin Sprocket and Gear offers a great resource for engineers — formulas for just about everything!
For our electrical engineers: Electrical Formulas (PDF: 476 K, 1 page)
See all of the PDF documents here.
Oh, and by the way, there’s also an app for that!
Check it out at the ITunes Store: http://itunes.apple.com/us/app/id369692504?mt=8