Valve's latest experimental Steam feature is Play Next. It's designed to recommend games for you to play from what you already own, based on what you've played before. It only recommends stuff you haven't played, so if you're one of those people that buys games and forgets about them it's a pretty good reminder of what you've got waiting for you in your library.
My Play Next page reckons I should give Borderlands: The Pre-Sequel a go, because I've played loads of Borderlands 2 and Risk Of Rain 2. I didn't even know I owned The Pre-Sequel so I suppose it's done a pretty good job scouring my library to find that one.
When you load up your Play Next page, it'll give you a selection of three games it thinks you should have a go at. If none of those tickle your fancy, you can reroll the picks and, from what I can see, cycle through all the unplayed games in your library. It says it uses "machine learning" to select the games it shows you; so far it looks like it's just matching up genres with stuff you haven't played to recommend them.
I don't think it works completely as intended, however, because it's also saying I should play Payday 2, as apparently I haven't played it and it's similar to Rainbow Six Siege. Except I've definitely played Payday 2. A lot. I just haven't played it for years. (Though now I'm checking, Steam says I have no hours in it at all, which is possibly a separate issue. That or I'm going insane and playing the heist game dressed as dudes in masks was a fever dream.)
Along that line of thought, I also haven't played Siege for ages because I got quite bored of it, so recommending me a game based on something I'm not really into at the moment seems like an odd choice. In the future it'd be nice to see recommendations based on what you've played recently.
The system is still a work in progress, hence why it's still a Steam Labs experiment, so it shouldn't come as too much of a surprise if it gives us some odd comparisons or recommendations at the moment.
Try it out yourself on Steam, and tell us what games it suggests that you completely forgot existed.
Steam Labs is a place for all of Valve's new experimental features to grow, letting players try things to see if they work before rolling them out as proper features. They've been busy in the past months with their recommendation engines, too. There's the Interactive Recommender which has made its way to the Store's front page now, and Talk Of The Town is another experiment in development to show you games you might like based off of recent user reviews.