Some time ago, I got an Everdrive GB, a flash cart for the Game Boy / Game Boy Color. As you might guess, it can play the vast majority of licensed GB/GBC games (all the Donkey Kong games for GB/GBC that I've tried, as well as numerous other games, work flawlessly), and is compatible with most Game Boy models, such as the original GB, the GBC, the Super Game Boy, and the GBA models (though I heard it has issues with the GB Pocket). Like many other flash carts, it uses a Micro SD card, and you can store several ROMs on there. (Hopefully these are ROMs that you obtained legally!)
Needless to say, it can run ROM hacks as well, so if you want to run hacks on real hardware (or test hacks that you have made), this is a great way to do so. (I was quick to try the English patch for the GBC version of Donkey Kong Land III, and it worked perfectly!)
Not surprisingly, it's also compatible with the Link Cable, so you can connect with other games such as Pokémon as if your flash cart were the real thing. I'm assuming that it also works with the GBC's infrared feature, but I haven't been able to try this.
Also, the save files that are stored on the Micro SD card are compatible with save files that emulators normally use, so that's a good thing.
Another thing that I like is that, unlike many other flash carts out there, the Everdrive GB is rather reliable, and won't break on you easily. There's no having to worry if it will stop working after normal use!
As good as this is, one major shortcoming is that it lacks a real-time clock. While only a few games use it, such as Pokémon and Telefang, they won't work properly on the Everdrive GB flashcart, so these games require patches to be playable here. (Couldn't the Everdrive GB have a fake real-time clock or something?) Other special features that are unique to a few cartridges, such as a motion sensor or a camera, are not supported either.
In addition, it does not support the N64 Transfer Pak, because N64 games won't see the Everdrive GB as an actual Game Boy game. Though, apparently, there is a way to mod the Transfer Pak if you're concerned about this, but I don't know the details of this too well.
Finally, I must mention that the price is rather steep -- it's roughly $80 USD, which is more than many new video games. The wide game compatibility, as well as the physical durability, make up for this, even with the glaring lack of RTC. If you're a Game Boy freak like I am, and want to play several games in one cartridge (including ROM hacks), then I would recommend this.
And if you're curious as to how the cart looks like, here's a picture of mine. Though, I've seen carts with different labels, so I guess it depends on where you buy it from.
Does anybody else besides me own an Everdrive GB?