A recent issue of Robot Magazine included a nicely done writeup on the merits of Oscilloscopes for robotics hobbyists. I totally agree.
The article got me to thinking about the tools I use. Not the obvious ones like Digital Multimeters or the cool ones like Bus Pirates or Logic Analyzers, but the tools that are indispensable yet are never the first to come to mind. They're the unsexy underdogs of the robotics hobby world...
001. Paper and writing utensil: Even after almost 2000 years, the convenience and immediacy of paper has yet to be surpassed. Having a big stash of scrap, blank paper and pencils/pens is awfully handy for capturing thoughts, doing calculations, drawing designs, drawing robots, and the like. The key is having a stash you can grab from lickety split.
010. Laser Printer: I've used it to create PCB transfers, test PCB sizing, make hardcopies of various robotics and electronics information for quick reference, and generate templates for drilling/cutting robot chassis, among many other things. My old Laserjet 4m+ is an industrial strength workhorse and should last another 20 years without any trouble.
011. LEDs: Cheap, simple diagnostic tool, like an in-circuit logic probe. Throw one on the protoboard to make sure the power's on. Throw one on a line that's supposed to be high. Throw another on the motor when it's on or use dual color to show direction.
100. Cordless Drill: Great for driving screws for assembly/disassembly, drilling holes, chamfering or countersinking, can also use for twisting wires together (put one end in the vise, chuck the other end, and begin).
101. Flux: Even with the best iron, solder work/rework is going to be poor without flux, which allows molten solder to flow better. I'd rather have flux than a good iron. The best choice is a flux pen like ones from Kester, but Radio Shack flux paste, while messy, works and is good at sticking SMD parts onto PCBs.