Steam launched on September 12, 2003, making it 20 years old today, and Valve has honored that milestone in the same way it always acknowledges the passage of time: with a sale. All Valve games are on sale today for the The Steam 20th Anniversary Celebration, as well as a few games from other publishers, and there are some free Steam profile accouterments in the Points Shop.
Valve has also turned the Steam store's background "army green" like it was in 2003, when it looked like this:
Back then, Steam was mainly a tool for Counter-Strike updating and matchmaking, but when Half-Life 2 came out in 2004, it had to be tied to a Steam account, and that really marked the beginning of the Steam store as we know it now. It also made people pretty mad: The idea that we needed online accounts to access games other than EverQuest was the scary gaming trend of the 2000s. 20 years later, people get mad when a game isn't available on Steam, so Valve did pretty well for itself.
Steam introduced a lot of conveniences, too, like auto-updating and matchmaking features, and ultimately led the gaming industry's transition from discs to downloads whether grumpy old physical media-loving PC gamers liked it or not. It took years for consoles to catch up, hence why we use "downloadable content" to refer to small downloadable expansions even when downloading entire games was already the norm on PC when the term was invented.
For Steam's 20th birthday, Valve has put together a timeline with some other fun bits of Steam trivia. Right at the start there's the reminder that fangame Codename: Gordon was technically the first third-party game on Steam. The 2D Flash game was produced by Nuclearvision, a developer that no longer exists, and Codename: Gordon isn't listed on Steam anymore, but you can still download it by entering the following URL into a browser: steam://install/92.
The timeline also notes that the first big Steam sale was December 2007. Today's 20th Anniversary Sale isn't on the scale of one of those seasonal Steam sales, but a deal that sticks out to me is Mass Effect Legendary Edition at its lowest price ever—just $11.99 for all three Mass Effect RPGs—in case Starfield's release has gotten you in the mood for more interstellar adventuring. Keeping with that theme, Elite Dangerous is just $7.49, which isn't quite its lowest price ever, but is close.
Almost every Valve game is on sale, too, so if you haven't played Portal or Portal 2, you can grab them for a dollar each. Valve puts its own games on sale all the time, though—check our Steam sale dates guide for info on the next big seasonal sales.