Steam
Used by 27 peoplestore.steampowered.com ↗

“It sits in my homelab and runs headless so I can stream games to other machines in the house via Steam Remote Play.”
Katherine Cox-Buday Interview, Writer, computer scientist · via Uses This 
“PC gaming on the couch is full of hardship, though worth it for the Steam sales and the many indie games that only come to PC.”
Max Howell Interview, Developer (Homebrew) · via Uses This 
“I do a little bit of gaming and I have a Windows-based Puget Systems PC that basically just runs Steam.”
James Turnbull Interview, Engineer, author, CTO (Empatico) · via Uses This 

“My Thinkpad does keep a Windows partition around for the occasional Win- or Mac-only game, but Steam's surge of Linux support and Humble Bundle's years of insistence on Linux support has made Linux support so likely for new games that I could probably ditch this now.”
Eric Mill Interview, Web developer (18F) · via Uses This 
“Steam: I don't play very many games but Steam really makes it easy to buy/install games without having to have physical discs or worry about copy protection being installed.”
Justin Frankel Interview, Software developer (Winamp, REAPER) · via Uses This 
“I'm also an extremely heavy user of Steam for purchasing the numerous games that I never play.”
David Cramer Interview, Developer, infrastructure nerd (DISQUS) · via Uses This 

“A decent fraction of the last 10% may or may not be Steam, but, if it were, I'd say its fraction has been cut down a lot since my daughter was born in April.”
Chris Slowe Interview, Chief scientist (Hipmunk) · via Uses This