Park Tool SPA-2 Spanner Tool
The SPA-2, will fit one-key release retaining rings on Shimano and Campagnolo cranksets & the adjusting cone on most freewheel clusters (not freehubs). Adjusts to a wide range of pin hole widths. Pin diameter is nominally 0.090" (2.3mm).
This is a very specific tool. Andrew G kind of nailed it. If you need one and don't have it, you'll be bummed like crazy. The frequency with which you'll need this, though, is very low. Does its job beautifully.
I use these for my old iDrive. Works pretty well. Not much to say other then it waits to be used and then it works.