I use this one. It works in Linux and Windows with no problems (nowadays anyway).
It has a weird firmware bug that doesn't affect windows (and Linux was patched to get around it) and also doesn't affect ScummVM (on any OS - ScummVM was patched to get around it). It may affect OS X with DOSBox though. Cannot say for sure since I don't have OS X.
Thank you both for your replies. I can't argue with the price of the one on Amazon. The RoMI/O II seems to be quite a bit more expensive from what I can tell, but thanks for the suggestion balpat.
Honestly, I would probably take the one from Amazon as well. I thought it might be a good idea to have two MIDI Interfaces because I had originally planned to buy an additional MT-32 to my CM-32L but there's really no reason.
I think that probably all these USB-MIDI devices will work.