The Belkin F5U119E works quite well. My Unicomp Customizer had issues with the "blue cube" type adapter but worked almost-ok with the Belkin. Very occasionally it would suffer a stuck Shift key during gaming, but that never occured when typing normally IIRC. This was through a KVM switch too - an added complication.
I wish that I still had it so that I could test it with my SGI AT101 (with which the blue cube fails) but I sold it some time ago (either with the Unicomp board or my old KVM IIRC). The blue cube is a fairly safe bet, unless you have the SGI AT101 (the Dell AT101W works fine). My Unicomp Customizer (105-key ISO model with Windows keys) worked almost okay, but not sufficiently well to use day-to-day.
One thing to watch out for the lack of support for the additional key on ISO keyboards. I originally bought a cheap unbranded beige coloured adapter from eBay and it worked very well except for it didn't support the \ key at all. Both the Belkin and the blue cube had no issues with that. Some of them can be switched to support ISO layouts from the default ANSI by entering a special key sequence, but I could never find the commands to type in on my model.