What tools do you find most useful?

I absolutely love my little Gerber Clutch multitool. The nail file thing is great for pulling bridge pins. The wire cutters are perfect for snipping strings. It's compact enough to keep in my gig bag. It's cheap.

Gibson Les Paul BFG Gator
Line6 James Tyler Variax JTV-69
Line6 Pod HD500
Blackheart Handsome Devil
If like me, you'll ever have the pleasure to mainly play around with old and heavily abused equipment, you will learn to appreciate duct tape.