Victorinox Swiss Army Pocket Knife
For as long as I can remember, my dad had one of these in his pocket at all times and as I got older, I realized how often they come in handy. Don’t get me wrong, on certain occasions I like carrying a larger blade, but you can't beat this little guy’s functionality for your day to day carry. I use it to open letters, trim my fingernails, cut loose strings off clothing, tighten the occasional screw, and most importantly, it cuts the best hole for shotgunning beers. Also see the slightly larger version in the second link below.