After a rather short beta period, the Steam Link application for the Raspberry Pi is now officially out.
From the Valve PR:
The Steam Link App is now available free of charge for the Raspberry Pi 3B and 3B+ running Raspbian Stretch. The app, which debuted in beta last week, allows gamers to stream their Steam games library from their computer to other areas of the house via the home network using these Raspberry devices.
To install it on your Pi, you simply run these very quick and simple commands:
sudo apt update
sudo apt install steamlink
After that, you can then launch it directly from the Games menu or run "steamlink" in a terminal.
Valve strongly recommend a wired connection, according to them "The wireless module on the Raspberry Pi appears to occasionally have delays of up to 250 ms".
You can find out more about the Steam Link application on the official Steam page for it.