This emulator is for browsers on computers and will run on most devices with little problems. There are quite a few games ready to play straight away (no need to download to your device), but can't be used offline. It's ready to use straight away with no downloads required.
D-pad: Arrow Keys
This emulator is for those people who have Chromebooks (BLEGH) and want to do something fun without needing internet. It allows ROMs to be imported (so you can play them from your device if you download them) and can easily be used offline. It's technically a Chrome Plugin, so it can be used on any device with Chrome installed. You'll need to download the emulator and the ROMs, but it's definitely worth it.
Open Rom: Ctrl + O
D-pad: Arrow keys
Save State: Shift + F1-9
Load State: F1-9
Reset: Ctrl + R - Speed up: Space
Screen capture: F12 (syncs to Drive)
Vol down/up: -/=