Soldato
The Stanley Parable
After getting and selling a lot of Steam cards I had a free (I guess?) £2 in my wallet and finally picked up this game. It was fantastic! It's been a while since I enjoyed something this much.
Granted it is short, there are 19 endings, and has zero reply value once you have explored all aspects of it. As a game it is very different and I'd highly recommend it if you want something unusual.
I'd quite like to play something else quirky like that now.
Try Pony Island. I've not played it myself, and have avoiding finding out much about it before jumping in but, from what I do know, I reckon it's right up your street if you enjoyed SP.
Antichamber might be worth a look too. I have played that one, and it was pretty smart.