In the MCU they're called the Infinty Stones, but in the comics they are Infinity Gems and one of the most powerful is the Mind Gem. This blue gem grants the barer psionic powers like telepathy and telekinesis and can augment any pre-existing mental powers. When combined with the other gems, it allows the user to connect with every mind in the galaxy at once. Great for viral marketing. On Earth-616, it has fallen into the hands of Adam Warlock, Beast, Grandmaster, The Hood, Moondragon, Nebula, Professor X, and of course Thanos.
In the films, it starts off in Loki's scepter that he got from Thanos and allowed him to control minds. When Loki was defeated, it fell to SHIELD, where it got moved by HYDRA agents inside SHIELD to Baron Strucker who used it in experiments… with only Quicksilver and the Scarlet Witch coming out alive. It was then retrieved by the Avengers and Tony Stark used it to create the AI that would become Ultron. So, not a good move there. Ultron would use to to help create his new perfect body, which he never got to use, and the stone now resides in the forehead of the body which is the Avenger called Vision. Though it was blue in the scepter, the mind stone is actually yellow. Which conflicts with the comics, but no one really cares.
The latest Marvel 101 takes us through the comic history of the Mind Gem / Stone briefly, showing us how it's been used over the years and by whom.