This is the answer to those times when you’re pushing hard on the shifter hoping beyond hope that there’s one more gear left. Offered up by Complete Concepts, it slides onto freehub body behind the cassette, then you remove the 2nd or 3rd smallest cog before tightening things down. There are a few more steps to it, all outlined on Australia’s BikeExchange classifieds website, where you can pick one up for AU$65. It only works with Shimano 9-speed cassettes. You can also find it on eBay.
Hat tip to Brett for the link!