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Initially released in September 2003, the digital PC gaming platform developed by Valve named Steam has soared in popularity as it has become the primary place to purchase PC games, ruling over other platforms like Origin, Games for Windows and Impulse Driven. Though it initially was exclusive to Windows, it now has a Mac port and will soon have one for Linux while a modified version is found in the PlayStation 3 version of Portal 2 and in some schools for critical thinking purposes.
Because the library has thousands of titles to pick from, Steam is easily the best place to look for a new PC game. There are not very many titles that are not on Steam, and the primary ones that aren't are developed by Electronic Arts, who owns the Origin platform and therefore makes all of its releases exclusive to it. Other games that one would not find on Steam include older games, which are typically found through GOG, though older games have been making an appearance through Steam.
For newer and larger games, Steam has just about everything for its millions of users. Its popularity can be proven by viewing the Stats page from its website at any given time; there are always over a million people playing just one game through Steam, with roughly a million others collectively playing other games in their Steam libraries. This is far more users than any other platform has even registered at the current time, which ultimately proves Steam's superiority in the market.
Valve knows this but doesn't abuse it, going as far as to make great deals every midweek, weekend and holiday, where games and packs are up to as high as 90 percent off. With all of these factors taken into consideration, it's no wonder why Steam has remained in the top spot for so long.