[Error: unknown template 'qotd'] I guess I'd have to say my Leatherman, even though it's strictly not a 'tool of my trade'. It's more of a Tool For Life. Being the girlfriend of a certified nerd, I had to get myself one of these, and when I found it on special somewhere, it would have been foolish not to get one.
I have bought a nice sleek black leather sheath from Victorinox, that it fits in very snugly.
My Leatherman is a Wave of the latest model, which appears to give me instant geek cred. But it's actually very useful in other ways, too.
Here's a list of all its features:

* Needlenose Pliers
* Regular Pliers
* Wire Cutters
* Hard-Wire Cutters
* Clip-Point Knife
* Serrated Knife
* Saw
* Scissors
* Wood/Metal File
* Diamond-Coated File
* Large Bit Driver
* Small Bit Driver
* Ruler (8 inch/19cm)
* Large Screwdriver
* Bottle/Can Opener
* Wire Stripper
* Lanyard Attachment

Impressive, huh. I do miss the corkscrew every now and then.