Press and hold Shift and the right mouse button and type very quickly:
Psychonauts: Cheat Codes Free Download
All Psi Abilities: X, X, F, E, Tab, F.
All psi abilities improved: mouse wheel, spacebar, Tab, Tab, F, X.
Rank 100: Tab, mouse wheel, Tab, E, X, E.
All inventory: mouse wheel, X, E, E, Tab, F.
9999 Lives: Tab, E, E, X, Space, Mouse Wheel.
9999 psitanium tips: space bar, mouse wheel, mouse wheel, E, F, left mouse button.