I've used an index peg mounted in a wood fence attached to my miter gauge. It was a pain to dial in and only good for one size box joint. It either needed a dedicated miter gauge or required careful setup if I removed it to use the miter gauge for another operation.
Then I made this standalone design from one of the WW mags. It was adjustable and had replaceable sub-fences with pins for different size fingers:
I took great care building so it looked nice, but it really didn't work worth a darn! It was hard to get the dado stack to match the index peg from use to use.
Then I started over and built this really crude prototype. Outwardly, it may look like other adjustable box joint jigs, but I came up with a totally different principle that hadn't been used before:
Surprisingly, it actually worked quite well but was certainly not ready for prime time. So, using a similar mechanism, I built this one from acrylic:
It worked much better but was still lacking, so I built this one from an aluminum extrusion:
It looked really slick, but since I don't have metal working tools, machines, or skills, it didn't work worth a darn. So, I teamed up with the great folks at INCRA and we came up with this:
It is the cat's meow! Among other things, it is precise, micro adjustable in .001" increments, easy to use, works on tablesaws
AND router tables, can be set to make a wide range of fingers from 1/8" to 3/4"+, and can make a number of specialty joints not possible with other jigs: