Well, they announce it on Twitter that they are going to release codes for the next few hours. I remember I once camped all night for that. Still didn't get any codes. (My first attempt ended in failure: I hadn't played Spiral Knights in a long time, so when I got on Spiral Knights to redeem the code, it started patching...) Sometimes they even have contests, like a Haiku contest, with prizes being codes. These Twitter prizes included a variety of random accessories (One was a flower), love auras, and love puppy costumes.

The next one that comes to mind was Black Friday. They had 500 codes on select websites, and the only website I found that didn't require an account had Regal Parrying Blades. All of the select websites had some form of parrying blades. I went on another computer to nab a code to see if I could redeem a second code, but I couldn't. I gave away that Spare Code to a friend whose friend missed out on the promotion.

The last one that comes to mind was the PAX bombheads (Known as Prime Bombhead Masks in game.) They handed them out at PAX and I have heard tales of people coming across that really friendly person who ran the booth and gave away dozens of codes per individual. One guy started posting some on the forums for those lucky enough to redeem them first. I think he got banned eventually. I forgot.

All of those redeemable codes are one time use only. As of now, I'm fairly sure there is no event that is handing out codes.