Valve has opened up the current version of its Source engine to modders, bringing several shiny new features to play with. Chief among these are native support for the famed Oculus Rift cybergoggles, and the ability to create Mac and Linux versions of Source mods.
On top of those, the Source SDK 2013 also adds support for Steam's new SteamPipe content delivery system, which brings shorter loading times for games and maps plus easier modding of mods. For mods distributed directly through Steam, which is a rare sight nowadays, it'll also allow for faster downloads.
Source SDK 2013 also changes how tools and code are distributed, shifting the code to GitHub with a new license which Valve says "allows mod authors to share their changes to the SDK more easily." Check out Valve's instructions to get up and running if you're making things.
Do bear in mind that modders will need to update and release new versions of their creations to use these shiny new features: it won't happen automatically.