Sure, you already own Psychonauts, but here's a chance to buy it again, as part of the Humble Indie Bundle. Why would you want to do that? Well, it's the whole charity thing, but more importantly, the other games in the bundle are pretty much stone cold classics. Limbo, Sword & Sworcery EP, Amnesia: The Dark Descent, and (for a $7.34 donation at the time of writing) Bastion? Oh! Em! Gee! to that! Plus you get all the soundtracks thrown in too, all for an amount of your choice!
The games are available on Mac, Linux, and PC, and there are few excuses not to do this. Run and donate!