Hey, my first article is on the various promotional cards Kongregate has offered. These cards feature different art, but ingame, they're the same. They are normally released for holidays or events, so stick around and you might see a few.
PAX Tafari is currently the only Promo card you can still get. At the PAX convention, Kongregate had a booth, and gave out real live PAX Tafari cards. They had codes on the back which allowed you to unlock the ingame version. They have extras and are giving them out here
Halloween Blood Vial:
The Halloween Blood Vial was released on Halloween 2007, for one day only.
Valentine's Day Gem:
The Valentine's Day Gem was released on Valentine's Day 2008, for one day only.
The rickrolled breastplate was the result of a funny prank by Kongregate on April Fools' Day 2008. They pretended Kongai was launched, but instead you got rickrolled. The challenge that day was watching the whole thing to get the Breastplate.