Below you'll find a relatively regular updated code list. Codes last 7 days once they're released. Codes generally appear at the start of an event, after large game updates, and very rarely after small patches. They will appear in-game via a yellow flashing exclamation point (!) in the top left corner of the main battle window. They are redeemed on Daily Rewards tab.
You can also find codes by checking the Kongregate forum post here.
Codes have been discontinued since the 6th of December.