You guys may laugh, but for me it's my Swiss-Tech utility key.
I also carry a Leatherman Wave and a BM 550 as part of my EDC, but the Swiss-Tech is the only cutting tool that can go anywhere- and I mean anywhere.
I have used it to repair my bicycle, assemble IKEA furniture at a party, fix strangers' glasses and PDA's, as well as cut innumerable packages.
That's versatile in my book.