A De-burring tool. If you're doing any drilling, give this a quick flick around the freshly drilled hole to clean up any mess- Especially handy when drilling plastic enclosure lids.
They're also pretty good for slightly enlarging holes when you haven't got quite the right drill bit.
A fountain pen. I'll use a ballpoint when convenience demands it (i.e, on site, writing on non-paper surfaces), but a fountain pen is more or less mandatory when I'm at my desk.