My Leatherman Wave pocket tool.
http://leatherman.com/products/tools/retired-tools/wave/
Goes with me just about everywhere. It's opened many a 35mm canister, cut off many a leader or trimmed it to be useable in a barnack camera. It's tightened and loosened many tripod mounts, opened and closed camera strap lugs, if held just right with your jaw screwed in just the right position, can be a lack-of-anything-else spanner wrench, opened up just about every joyful photo purchase I've gotten from UPS, Fedex or the US Postal Service. In a pinch, it's even cut processed rolls of film into strips.
I know what it will do, it never fails to do what I ask of it, it never presents me with any surprises.
I went through serveral of its siblings earlier in life. I'd show them off to friends, wear them as belt candy, lend them out for others to use. Ultimately, I'd take them for granted, use them, misuse them, and invariably leave them behind somewhere like under a machine room floor or fallen unceremoniously out of my pocket under a commode on the cold tile of a public rest room floor.
This one came to me with much anticipation. It's the second Wave I've owned. The first one was too loose from the start, and that was a dangerous proposition. This one was just right. I make sure it stays that way by washing it and rubbing it with oil at regular intervals and wiping it clean and dry with a cotton cloth. I never stress it more than I should, and it's thanked me by serving me well. I like to think I'll keep this one until it won't have me anymore.
-KwM-