There are only two of that I know about: the NeoFlash, and the Tototek.
The Tototek uses a parallel (!!!) interface to connect to your computer, so if your computer was made in the last 10 years, there's a good chance you won't be able to use the Tototek. Another drawback is the ones I've seen are basically just a big giant exposed circuit board with pins on one end you stick in your console's cart slot. That's really all I know about it; I don't have anything with a parallel interface, so I've never bothered with one. Even if I did, I'm not a big fan of big, giant exposed circuit boards.
The NeoFlash is what I've got, and it's a pretty good product. The build quality is great; it's the size of an actual HuCard, with no exposed chips or anything. It uses a USB interface to connect to your computer, which is the way it should be. The NeoFlash's big (only?) drawback is that it's been determined that it has weird issues when used on consoles with a weak +5v line on the cart slot. Unfortunately, there's no way to really know if your console is one of these until you buy the cart and try it out. I've got a bunch of NEC systems, and out of them all, only my U.S. TG-16 seems to have this issue (although it's been reported by many other people as well, and it's not always U.S. consoles). I would imagine if you've only got one NEC console, and you buy the thing and it turns out your console is one of these, you'd be pretty bummed out. I don't think anyone has a way to fix this yet.
