6. The Infinity Gems
Of course, the Infinity Gems (or stones, as they're known in the Marvel Cinematic Universe) were explicitly mentioned in the mid-credits scene of Thor: The Dark World when Benicio del Toro's Collector came to Asgard to pick up the Aether - which we found out is, along with the Tesseract, one of the MCU's adaptation of the Infinity Gems. However, the keen-eared amongst you will have heard a subtle reference to them earlier in the movie. When Odin is explaining to Jane Foster what she has inside her (the Aether), he explains that it is a liquid-like object and says "they are usually stones, but this one is different." This, of course, was a reference to the fact that the objects that fit so perfectly in the Infinity Gauntlet - the Infinity Gems/Stones - are normally solid objects but the Aether, despite the fact that it is one of them, is not.