There isn't one "accessory" in my knitting arsenal that is more useful than my swift and ball winder!
They were the best thing I ever gifted to myself!
Next most useful: professional blocking boards, and blocking wires!
I put them in "2nd most useful" cuz you can make your own blocking surfaces with other materials, which I did for years. Always had to 're-make' them cuz they never held up well.
Third most useful: the OTT lite! You can knit til the wee hours with the OTT lite over your work!