For me, it's the BandAids! I'm on my second box. Did I mention I'm a klutz?!?!
The "surprise tool" of my build was the small block plane. I have used it more than I would have ever expected. Especially when I needed to shave 1/8 or so off the paneling to make it fit.
The "cool tool" was the Kreg pocket hole jig. I didn't even know what a pocket hole was when I started my build. After hearing about them here I went out and bought one. I've used it for everything.
The "best return on investment" was the Veritas sharpening jig. It paid for itself! Rather than buying new chisels and planes, I bought the sharpening jig and reconditioned all the old tools.
The best "time-saving tool" was a second drill. I used one to drill and countersink, and the other drive the screws. It's nice to have friends with tools. (Another friend let me use his power miter saw. It's nice but a little scary to use.)
Do the Dutch Ovens count? I have to eat while I build.
Ira, I would have thought your most important tool was your ICE CHEST!

Gotta keep that beer cold!
Joanne
Still can't figure out how to get the router to do the miracles I see you guys doing with them.