Flash games made just for the fun of it.
The younger generation probably doesn't even remember, but the 2000s were the era of flash games. There were so many games created and practically none of them had in-game purchases. Nowadays, every single flash game has the main agenda to get you to buy it's in-app purchases. I miss the days of Defend your Castle, Stick RPG, etc..
edit: I'm a huge fan of good flash games, here are some I recommend that aren't about in-app purchases, but are fun and challenging: Rebuild, Motherload, Monsters' Den, Creeper World, Battlecry. Kongregate.com is still a great site to find them on.
e2: Okay, sorry I'm being that OP that keeps making edits when his comment gets popular, but… I love you guys. I miss old school flash gaming, I even tried picking up ActionScript3 a few years back and stuck with it for a long time, even though people constantly said "flash is dead/dying" w/e. Makes me super happy to read all these comments and all your suggestions for great flash games. Keep the dream alive, yo.