Files found within Valve's Source Filmmaker program reference a next-gen engine from the company, referred to as Source 2.

Numerous lines of code reference Source 2, as uncovered by Valvetime.net, suggesting the new engine is in development and primed for inclusion within Filmmaker at a later date.

"Return an str with the current engine version. If key doesn't doesn't exist, assume 'Source', otherwise invalid -- assume next-gen 'Source 2'," one snippet of the Matrix reads.

The original Source engine has proved an adaptable and long-lived beast. It originally debuted with Counter-Strike: Source and Half-Life 2 in 2004 (yes, it really was that long ago).

More recent Valve games have included newer iterations of the engine, dotted with various improvements such as better facial animation and support for different platforms.

The above code suggests that Valve are working on a new and entirely separate Source build for use in future games. Maybe even Half-Life 3.

Valve officially launched Source Filmmaker last month on Steam. The free software allows users to create movies within the Source engine, using assets from games such as Team Fortress 2.

"We don't like to brag, but Source is considered the most flexible, comprehensive, and powerful game development environment out there," Valve states on its official site. "And it's about to get even better."

http://www.eurogamer.net/articles/2012-08-06-valve-files-name-next-gen-source-2-engine

I think they just bragged a little (they should)

At long last! Seriously, it was time for an upgrade.

Now the thing is, with Valve's movements to Apple's OSX and now Linux, will the new engine be compatible with those OS, giving developers an engine that allows them to make games that run on every PC whatever the OS they use?

I predict it will be OpenGl.

Given the recent news about Linux and OpenGl, yes, it will probably use it.

You're right, kinda obvious 

The question is if it will take advantage of OpenGL 4.3.

http://www.brightsideofnews.com/news/2012/8/6/khronos-announces-opengl-es-30-and-opengl-43-and-more!.aspx

Given that the only engine that uses OpenGl is IdTech5, and that now it's an in-house only engine, I'm quite confident that they will try to convince Valve to support it.

Unity, UNIGINE Engine, Gamebryo and a few other indie engines support OpenGL

That's are the key word, indie.

I was talking mostly about known, mostly used engines to make big games. Or to put it like Anandtech (from which I took that it was the only engine using OpenGl):

"On Windows OpenGL usage has never been lower; the only AAA game engine still based on OpenGL is id Tech 5, which with the termination of licensing by id is only used internally. Khronos of course would like to change that, so when Valve Software says that they’re going to be porting Source over to Linux (thereby increasing the audience for OpenGL games) it’s of quite some interest to Khronos."