*edit* Yeah, I see it's been mentioned a few times before in the thread, but here's my reinforcement of this one being hard to find.
I recall back when Skullmonkeys launched on Playstation (still one of my favorite platformers of all time) I had a HELL of a time finding a CD-Rom copy of "The Neverhood" on PC (a point and click adventure/prequel to Skullmonkeys made by the same team who did Skullmonkeys featuring the Klaymen character)
There are copies on eBay going for OVER $200 OPENED ... so, I'd say that's a rare and sought-after PC game that continues to live up to the difficulty that I was having finding it over 10+ years ago.