Uh, so this kinda came out of nowhere, but the Source 2 engine has been shown off in a post on NeoGAF by 'CBOAT'. The user posted a screen cap from what appears to be a leaked Valve slideshow.
The slideshow shows off Source 2.0, which is Valve's next-generation graphics engine. The presentation in question shows Left 4 Dead 2's plantation level, rebuilt in the new engine. If this is real, it might open up older games to be re-painted under Source 2.0, which would be incredibly awesome. We should hopefully hear and see more info on Source 2.0 as time goes on, especially with the lead up to E3, and of course SteamOS, Steam Box, Steam Machines and more.