If you are fair with text-based games, Anchorhead can be gotten for free (along with a z-machine interpreter). It is not only one of the best interactive-fiction titles out there, but an excellent Lovecraft pastiche with elements from a number of stories that fit well together. There are a number of other free games in this medium.
The original Slender works well on Mac and is free. http://www.parsecproductions.net/slender/
Besides that, the only ones I've played have been more in the "cheap" than "free" category. Stuff like the original Alone in the Dark trilogy (which is like $5.99 on GOG) and stuff that comes through the various Humble Bundles.
There was/is an official page for it, though I don’t remember the URL. You could search “Slender Official” or something, maybe that way it will appear.
Edit: here it is http://www.parsecproductions.net/slender/
> A $28 million budget for a shitty supernatural horror movie is absurd. Blumhouse could've made this for $3M.
It's especially egregious when you consider how successful the <em>game</em> was, on such a low budget.
Wait, bugger, missed the Steam part. Slender is a simple, free to play indie game, and it is frankly terrifying. Here is the website if you want to check it out. Should run well on most machines, and again, it is terrifying.
If you choose to play this, I shall say again - Good luck.
assuming that you don't have a very high end pc since you since you said you don't game that much, i recommend you download slender the 8 pages. its free. or if you don't mind paying you should get five nights at Freddy's. there's 4 of them and every new game is scarier than the last. both of them are horrifying and can ran on a toaster.
slender: http://www.parsecproductions.net/slender/
five night's at freddy's (you have to download a different free program before you can buy this and play it) http://store.steampowered.com/app/319510/
The game pictured is a game called 'Slender', it's a survival horror game where you are lost in the woods, being stalked by the Slenderman. There are pieces of paper scattered around in various locations (you need to collect 8 pages to 'win'), and the more pages you collect, the harder it gets. I think it's a great game, it has an eerie atmosphere and it's a good creepy game overall. (I like it better than Amnesia). It's free, so you should check it out! You can download it here.
Also, if you do choose to play this game, i just thought I'd let you know: Your flashlight has a limited battery, so don't leave it on all the time. There's nothing more scary than being stalked by a paranormal creature in pitch darkness with nothing more than a dying flashlight. ;)