Valve has integrated game streaming and viewing into its own digital client with the new feature Steam Broadcasting.
Watch An Hour Of Halo 5: Guardians Beta Gameplay
When a friend is playing a game on Steam, you can simply select him or her and click the new 'Watch Game' button, opening up a window into the live gameplay. No additional apps or fees are required to either stream or watch in this manner.
Valve states that the Steam Broadcasting beta is currently open to everyone who wants to use the feature--you merely opt in using the Steam Settings menu. It is of course still a beta, and Valve warns that "concurrent viewing may be limited" as they scale up the service. Valve also provided an FAQ, which can hopefully answer any queries you may have about the service.
Destiny Expansion The Dark Below ExclusivePlayStation DLC Revealed
Valve's service would seem to be a simple alternative to Twitch, but the service is only in beta and does not have feature parity with the game streaming website. The Steam site or the Steam client's Community tab can act as a hub for finding streams, and it works with Google Chrome and Apple's Safari.
There doesn't seem to be a way to monetize streaming right now, which removes that incentive for the big Twitch names out there who make money from their viewership--it remains to be seen if that's a path Valve wants to travel down. If so, the end result might just be streamlined and simple enough to take users away from Twitch as eSports and other game streams continue to rise in popularity. Just don't be surprised when Valve streams its Dota 2 tournaments through Steam Broadcasting.