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In term of new game ideas: Portal 1 in 2007. All other titles were sequels.
In term of new game ideas: Portal 1 in 2007. All other titles were sequels.
Portal was a reimplementation of Narbacular Drop after Valve hired the developer.
It's their strategy: they also hired developers of Dota, Counter-Strike, Team Fortress, and even Ricochet.
The Portal team was aquahired, like dota2's IceFrog.