It's a very innovative and engaging 3D adventure game with platform elements.  The characters, dialogue, and detail is above and beyond most other games that I've played.

And you can download a playable demo:

Psychonauts Download Links Page

The demo actually contains a surprisingly large amount of gameplay value.  It entails the entire first level of the game, which is pretty challenging.  It's a 400mb download so you'll want to have a fast connection if you're going to go for it.

The game isn't sold in stores anymore, but if you look hard enough you should be able to find other ways of getting it.  I understand it's got quite a dedicated following, but it wasn't heavily advertised so a large percentage of people don't know about it.

Hopefully you'll get some enjoyment out of it.

Be Nifty!