I did a little research before buying, but it seems, not enough. I knew enough to get one with Bluetooth V1.5 (they're up to V4.1 at least now), since that seemed to be what was needed for LeafSpy. I ended up getting this one, for $11:
Below is the more expensive one (by about $4) that it may be designed to look like:
When I went to plug it in, lo and behold I found an OBD II dongle already plugged in! Well, that's a bit of good luck, because presumably it is known to work with LeafSpy, and mine wasn't necessarily going to work. Indeed, after a bit of trial and error and some help from the seller (of my Leaf, not the OBD-II dongle), I found that I could get LeafSpy-Lite working using the dongle already in the car, but not the cheap one I bought. (I paid $11 for mine with free postage, it's since gone up to about $13). LeafSpy Lite is the free, limited feature version, recommended to get your dongle working. [ Edit: I'll be posting the Leaf seller's dongle back to them today. ]
A little more research indicates that for extreme cost cutting, the cheapest models don't have a separate PIC processor for handling all the commands required for LeafSpy; they squeeze the code into the Bluetooth processor, which has limited flash space. (I can't find the reference for this now; frustrating.) It seems that some clones don't even use a proper ELM327 chip, and emulate parts of the real chip in a tiny, cheap microcontroller. As you can see, there aren't many chips in my cheap version:
You can see that the Vgate model is packed with more electronics, and even a larger antenna. The cheap one has no parts on the other side of the PCB (the side you can see through the translucent plastic case). The plastic case is much large than needed for the electronics. This is a nuisance, since in the Leaf, if you don't use an OBD extender cable, the dongle points downwards under the steering wheel, and people have reported knocking them out while driving.
So the take-home message is this: don't do as I did

I've ordered another dongle, for about $25.50 this time, which is much shorter, and according to my slightly extended research should work. I'll post the details when it arrives next week.