By now, the Gerber Dime is an EDC classic. It's small, lightweight, cheap and packed with a handful of useful tools, including a nifty bottle opener. One of my favorite tools inside is a box opener (took me years to realize that's what it was).
Currently, on Amazon, you can pick up a Dime of your own for as little as $11 (in purple). Prices for this deal vary by color.