We used to self host HTML based riddle games and just post them to i-am-bored.com for traffic. That was usually the extent of our marketing. In addition to the direct traffic, quite a few other archives crawled IAB for content so our links would propagate nicely. Then forum posts would crop up places like Nordinho and Facepunch by players posting walkthroughs or wanting to collab to solve something. From a single IAB link and absolutely no effort or monetary spend actually propagating the game, we’d end up with hundreds of thousands of daily page hits. It was pretty incredible.
Looks like that site isn’t around any more though. I guess people hit social media or the app stores when bored these days. Such a shame
Reddit though, is a close second. If you find the right board and manage to get passed the usually strict rules and monitoring bots, you can reach a huge audience and see reasonable traffic in seconds.