Hmmm.. Sanitarium :D

Omikron is pretty trippy.

Tyrian 2000 gets pretty trippy at times.
Also Earthworm Jim.

Hotline Miami will make you physically ill.

In a good way.

I don't know if the gameplay would qualify as trippy, but some of the clips I've seen from the Atlantis series (I think it was #2 in particular) suggest that some game dev was eating some kind of mushroom somewhere.

Evolva has the colours and weird evolving protagonists

Beyond Good and Evil has the anthropomorphic pig

Outcast for the expansive alien landscapes in its beautiful voxel engine

And even if its not full on trippy, Giants: Citizen Kabuto is certainly pretty insane

*edit*

and if I can include Steam then I MUST mention Cargo: The Quest for Gravity - WT actual F, Eye: Divine Cybermancy has a reputation for being pretty out there too but I've not tried it yet

ALSO this thread reminds me that the original American McGee's Alice isn't available anywhere online, that makes me sad, GOG should get it...