Every journey begins with a single step, even if that journey eventually turns you into one of the most powerful game development studios on earth, with the single most iconic storefront in PC gaming, a stable of acclaimed titles, an upcoming PC/console hybrid platform, and an e-sports tournament with a $10 million prize pool.
Yes, even Valve had to start somewhere. In 1996, before Steam Machines, before SteamOS, before Steam itself--heck, before Valve even released a single game--the studio had two projects in development: Quiver and Prospero.
Quiver went on to become the Half-Life we know and love today. Prospero died. It's now, for those interested, the subject of an eleven-minute documentary by ValveTime Database:
According to David Hodgson's Half-Life 2: Raising the Bar, Prospero was slated to be "a moody, literary game, drawing on sources ranging from Myst to Borges." Marc Laidlaw, who eventually became lead writer on Half-Life, was originally working on Prospero, a third-person exploration game turned MMO.
The ValveTime documentary claims that many of the features that eventually made their way to Steam were originally planned as part of Prospero: friends lists, server browsers, and user-made maps. All of this would be accessible through in-game libraries, similar to the ill-fated Myst Online or Portal 2's co-op hub. The video points out that some of Prospero's tech and ideas were implemented in Portal 2 an incredible sixteen years after the fact.
There are more details and previously-unreleased screenshots of Prospero in the video--this is the first time Valve has really opened up about the project since its inception seventeen years ago.