Still one of the best puzzle games ever made, Valve's Portal 2 now has a community-made VR mod available in the early stages that's already quite impressive. The mod works, although it's certainly a little rough around the edges with various bits not quite hooked up yet. Even so, it's impressive what modders are able to do with bringing games into the VR space.

From the mod GitHub it notes that single-player, 6DoF VR view, motion controls for the portal gun, grabbing objects and Workshop content work. And since the most recent update, the in-game menu now works too. They haven't hooked up the full UI yet though but it's playable enough to have a bit of fun with.