Interchangeable screwdriver.
Actually, I also love having the Leatherman multi-tool.
Screwdriver handle w/ 60 piece bit set
Ratchet w/many socket sets
Drill w/bits
Dial calipers (incredibly useful)
Scroll saw (for building gun racks)
I bought my first Leatherman back when they were first offered by Dillon Precision in their Blue Press catalog (a very long time ago). I wore that tool on my side for years, using it all the time.
Then one day, I made the mistake of using it on-site to fix a problem at a customer site, and the manager that needed to sign off on the ticket refused to do so, saying she wasn't going to pay for the service call, since I never even opened my toolbox to replace the bad modem in a PC that was always making noise from the speaker. She had a very bad attitude, so I went and spoke to her supervisor who did sign the ticket.
I never used that tool on-site again, without first pulling out a few tools from my box.
Mark