Your best free 2 play single player game? (FREE drug inside!!) EDIT: Added a new .GIF. :D

Super Crate Box - steam - SP http://steamcommunity.com/app/212800

Hearthstone - Blizzard - MP but with SP in the making.

Shadow warrior classic - steam - SP http://steamcommunity.com/app/238070

Thanks for drug it got me high... school musical... all 4 1 now when all twerk,twerk is donee...

check out the first couple of zork's and the hitchhikers guide to the galaxy text based games - free and will keep you entertained for many many days.

Sleeping Dogs... Far Cry 3... Batman Arkham Origins...
They are all good until I finish'em...

The Plan.

Swag

Hearthstone
Pirates Love Daisies

Swag

Tales of Maj'Eyal, http://te4.org/

It is a roguelike RPG, one of the best and 100% free. You can donate or buy on steam to unlock an extra class and a small inter character bank but that is it. Unlike most roguelikes it has graphics, does not hide any information from you, and good/configurable controls. There is permadeath but one of the modes gives you a lives system and with the amount of stuff can unlock death is just a chance to try something you unlocked.

Roguelikes in general are usually free, this one is just more user friendly and has the option to make it a bit less merciless. A few others might be Elona, Dungelot, and Desktop Defenders (Alpha). The last two are both designed to play a game in a few minutes.

For awhile free games were somewhat only found in flash, although many of those devs have since moved to mobile and/or put in microtransactions. Still many gems on sites like Kongregate, Newgrounds, and Armorgames. Elona, mentioned earlier, also has a base defence shooter flash game with a few roguelike elements. Name a genre and probably at least one good flash game of that type, some comparing with great console/handheld games of the same type.

If want a good flash jRPG I would suggest the Mardek series, both in story and mechanics it compares to ones found on NDS, or at least that is how remembering it. If want a bullet hell one of my favorites was Starwish which had branching conversations, ton of customization, and an above average story as far as they go.

While typing this thread playing a turn based dungeon crawler called Monsters Den Chronicles (3rd in the series), and planning on some Creeper World later (a unique RTS series that has both flash and standalone versions). And earlier in the week the latest Gemcraft TD, which is as addictive as ever.

The Free Bundle, http://www.thefreebundle.com/

It is mostly single player stuff, Dungealot is one of the games in a bundle there for example.

Rock Paper, Shotgun does a weekly feature on free games, so does Epic Bundle (A bundle aggregator)
http://www.rockpapershotgun.com/tag/live-free-play-hard/

This is the most recent one, http://www.epicbundle.com/article/freedom-friday-free-games-may-23rd-2014

http://en.wikipedia.org/wiki/Endgame:_Singularity

An odd, and old, game do not see mentioned often is Singularity. It is freeware. The story is simple you are a computer that gained sentience and have to build, upgrade, and hide from the world as gain money. I suppose the simplest comparison would be to Pandemic except you have much more control.

If want something akin to a RPG or base building game that will last you for months and play itself there is the recent rise of the Incremental game type that originally got popular with the release of A Dark Room, Candy Box, and Cookie Clicker. There were games before it, Anti-Idle being one of the best, but those took things more mainstream. Personally my favorite is Sandcastle Builder, but that one has a bit of a learning curve.

They all share one thing in common, you progress by clicking something alot at first then buy upgrades to add new mechanics and automate stuff. Eventually reaching the point that your focus is more on optimizing and directing development to unlocking something.

Late last year Starcraft 2 went free, all but the campaign, and with it the massive library of player created content of all genres so if you do not mind downloading it and making a Battle.net account that is a good option. There is even a MMO built on it that had done a kickstarter. Personally I felt the stuff made on Warcraft 3 was better but this is still good.

I could probably think of more but is what came to mind just now and covers stuff not mentioned elsewhere in the thread. (Edit: Wow did not realize it got that long, I usually ramble but manage to limit it a bit better. Sorry for no bolding or the like, do not know the commands.)

You Have to Win This Game is one of the greatest free games that I've ever played. I say free, as opposed to free 2 play, because there's absolutely no mtx in the game. Somewhat sad because that means there's no way to get the cards for it, but oh well.

Do you mean Free 2 Play, i.e. with microtransactions, or freeware? I don't think there's any F2P single-player games on PC, though there's plenty on mobile devices.

Underhell, A half life 2 modification, All you need is SDK 2007 (I think its that one) to play and it is one of the best games you will play, and the creepiest!

TrackMania Nations Forever. Same for multiplayer

This is a good place to check. Browse and sort by whatever, price, amount of players, etc... Sadly it has not been updated for quite some time, but is still a nice place to find something you might not have seen before.

Know what would make that image better?

Heheeeeeeee!

Iji

This. Not entirely free, though, because you still need the original game.
This one is free, you just need Source SDK Base 2007 on Steam (which is free, too) to play it.

Edit: Thanks for the free drug!

Barkley shut up and jam gaiden, the writing is beautiful.

Since it's still open... Serena, the best F2P SP (STORY) in Steam

Trackmania Nations.

Black Mesa, it was even one of the Best AAA-Titles last Year (and free)

Not my best but I really like A Dark Room, a great text based browser game.

cave story
edit:......what am i supposed to hallucinate...i was expecting slenderman or something all i saw were jaggy lines whenever i looked around anything reflective....if this is what being stoned is all about...well...