I dropped a crappy old screwdriver and the brittle handle broke. I was about to throw it out when I realized I'd used that tool for decades and the blade and shaft were still just fine, and it would be more in keeping with my making philosophy to spend an hour or two to craft a new handle. This is the sort of decision that makes no sense at all if you value your time at some arbitrary dollar value, but it makes complete sense if you enjoy making stuff and hate wasting anything. So don't try to tell me I could just buy a better one for $2; I know already. That's not the point. Move on.
Step 1: Design and Tools
This screwdriver had a shaft with a square cross section, so I couldn't do the usual rehandling trick of drilling a centered hole in a hefty dowel and epoxying the handle in. I don't have a lathe so turning a handle wasn't an option. I decided to make the handle in two pieces, rout a stopped half-depth dado in each one, and glue the two pieces around the shaft. I'd then use my router table to make the resulting square handle into something more comfortable to work with. I had a few softwood dowels left over from assembly of Ikea furniture, so I used them too, even though they didn't match the oak I made the handle out of.
The old handle was about 90 mm long, and I made the new one a shade longer (about 100 mm = 4"). The diameter of the handle matched perfectly the width of an offcut of oak I had lying around, at 19 mm (nominally 3/4").
Step 2: Stopped Dado
I routed a stopped dado in each side of the wood about 40 mm long and 3 mm deep using a 6 mm straight bit, which matched nicely the shaft dimensions. I then sliced off 10 mm pieces from each side on the table saw (care!), and test fitted them either side of the shaft. I'd cut the dado a little deep, so there was a bit of slop, but nothing epoxy wouldn't fix.
Step 3: Dowels and Glue
I drilled a couple of through holes in the handle for the dowels, then glued it all together with wood glue. I left it for half an hour or so then trimmed the dowels with a flush cut saw, and then sanded it all flat on all sides.
Step 4: Shape Handle, Epoxy and Finish
I used a bevelling bit on my router to trim all of the square edges to octagons. I then added a big gloop of epoxy to the handle and the metal shaft, left it to dry, and finished with polyurethane. My screwdriver is now better than new.