Psychonauts is a 3D action/adventure platformer set in a summer camp for psychics. In this future universe, a subset of the population has acquired incredible mental powers thanks to the arrival of the material Psitanium. You control Razputin, who is a child at the summer camp determined to harness his latent powers in order to become a psychic secret agent - a "Psychonaut".
While the game takes place at a secluded summer camp, there is great variety in the environments you explore because much of your journey takes place inside the minds of others, in worlds that reflect their personalities and subconscious minds. There are various items to collect during exploration in any environment, as well as an overarching quest you must complete to progress. Collecting these items increases your power in various ways. You learn new psychic abilities, as you become more powerful, that make combat victory plausible and puzzles solvable, as well as more complex.
On top of the great game design, it's awesomely funny, with great voice acting and a witty script. I just recommend you play it on a platform other than the Mac, because this version's erratic bugs and snail pace do not do justice to this video game classic.
Sam's Protip: Use your combat powers on NPCs for some priceless reactions.