8bitdo's Zero is a wee, well-made and thoroughly wonderful bluetooth game controller. I bought it by accident, thinking I'd found a surprisingly cheap wireless pad, only to learn (to my shrieking amusement) that its low price is caused by its low volume.
It really is tiny, at about 2.75" long and 1.5" high. It comes with a keychain ring and is by no means unreasonably large for this purpose: 
There's a D-pad, four buttons under the right thumb, two trigger buttons and, in the middle, a start and select button. A short USB cable is provided for recharging, but it doesn't function as a USB controller. Just wireless.
It comes in white with a blue or red back; the red one has a vaguely Nintendo-esque stripe across the front, too.
That is not only works but is well-made and durable makes it an absolutely fabulous stocking stuffer for people who play games -- especially given that it's compatible with virtually everything, including Android, iOS, Windows and MacOS. There's even a remote shutter mode for cameras, though I didn't test it.
It's not plain sailing all the way, though. Pairing it with my MacBook was easy, but it took a couple of attempts on my Windows 10 Zotac ZBox. Moreover, it lost its connection twice in a month, requiring unpairing and re-pairing to get working again.
Part of the problem is the inadequate instructions for shifting it between compatibility modes that apply to each system. Use the wrong sequence of presses and it'll show up as a keyboard instead of a joypad, or as some random zalgo gadget that does nothing except stay rudely paired with your PC.
Oddly, there's no button 3 in Windows joystick mode. It jumps from 2 to 4. So anything that expects it (e.g. MAME) will need to be configured likewise.
Finally, it is perhaps too small, at least for twitchy arcade play. I tended to find myself holding it like a harmonica to get precise control -- but in that position, the triggers are much harder to hit.
At about $17, though, who's complaining?
PROS: Tiny but well-made; Cheap; Works on everything. Tiny USB cable.
CONS: Too tiny. Finicky Blueooth. Crap instructions. Bluetooth only.
8BITDO Zero Wireless Game Controller for Android/ iOS/ Windows [Amazon]
