Compared to many on here I'm a hack. My two offerings are certainly not original, nor are they particularly well-executed, but they do work exceptionally well. I forgot how good this thread is, so reread the whole damn thing this afternoon.
I'm sure many of you oldtimers have made these same tools, but they're not in this thread, so maybe they'll give a new guy an idea or two.
First up is a clutch hub holder - very helpful for removing the clutch hub nut on a Harley four-speed if you don't have access to air tools. Someone chucked an old nail stake in my yard, so I repurposed it and welded it to a clutch friction disc. Just put the long end against the ground or the bike lift, and crank away on the nut to your heart's content - no spinning!
Item two is a sprocket socket. This is not as good as one of the ones from Jim's because it doesn't get sandwiched up against the sprocket, but it still works pretty dang good, and it's cheap. I split a cheapy 1 7/8" socket, and my buddy TIGged in a piece of pipe he had to extend it. (Stainless pipe and TIG welding. The labor is worth more than the socket!)
I also have a jig I use to work on four-speeds. I copied a buddy's pattern, just a big lump of steel welded to the bottom of an old trans plate. Bolt the trans to the plate, then chuck the whole mess into a vise so you can reef on it like a gorilla.