Technology Evolution and Impact

All of Valve's games use one of two engines of their own creation – the Quake-based GoldSrc or its successor, Source.

The Source Engine is a 3D video game engine developed by the Valve Corporation originally for their debut first-person shooter, Counter-Strike: Source.
Steam introduced a revolutionary service known as Greenlight, which allows indie game developers to post their games on the Steam website, and allow fellow gamers to play and rate their games for free or a small cost. 
Greenlight assists developers in testing their games, in addition to providing them a method to spread awareness and sales of their games.
Highly popular games featured on Greenlight are offered contracts with Valve to sell their games and have been given the opportunity to sometimes partner in production.

Mecha Ace - example of a game sold by the steam store.