A couple of years ago, "idling" in Team Fortress 2 for hats and weapons was a thing. Now idling is back, but in a different form -- a special app lets you collect Steam trading cards for games without playing said games.
If you're not familiar with idling, it basically meant standing around in a TF2 server, waiting for item drops to appear in your inventory. Some smart fellows then figured out your could trick Steam into thinking you were playing, when in actual fact it was just a command-line program imitating a proper server connection.
Eventually, Valve caught on, shut down the practise and even rewarded those who didn't engage in idling shenanigans with -- you guessed it, a special hat.
However, idling is back, but in another guise. Thanks to a program called "Steam Idle Master", it's possible to collect Steam trading cards without playing the associated games. Cards are normally awarded by playing a game for a certain length of time -- Idle Master effectively automates this.
The source code for the program is available on GitHub and going by the initial commit, has been around for about a year. So either Valve doesn't mind, or it hasn't seen enough widespread usage to warrant a crackdown. Honestly, it's the first time I've heard of it, though this could be the start of increased interest.