These can be anything: libraries, technologies, concepts, languages. By "favourites" I mean it's anything that you really like (not necessarily use), want to see flourish and continue to be supported. If these things were to go away, you'd be very upset.

Here are mine:

• OpenGL
  
• GLFW library - This one is really underrated IMO, it kills me to see people still use glut when GLFW is such a better alternative.
  
  
  
  • OpenRL - I'm really happy it exists and I hope it doesn't die anytime soon, even if it's not ready for prime time yet. The heterogeneous approach is excellent imo.
    
    
    
    • WM_POINTER* messages - I really like what they're doing with the new unified input stack library in Windows 8. It allows developers to treat touch, mouse and styluses as generic "pointer" devices.
      
      
      
      • C++11
        
        
        (I'm sure I'm leaving out many, but these are my top favs off the top of my head)
        
        What are yours? Feel free to repeat what others have mentioned if it's your favourite too.

Epoch.

MonoGame is piquing my interest lately as a way to allow XNA apps to be cross-platform. Bastion is one such game that took advantage of it for Mac and Chrome.

SFML
Qt

Lua
Recast and Detour
SFML

I found this thingy that someone here on the forums (AgentC) is working on, called Urho3D. I have only tinkered with it, dabbled really, but I would like to see it continue and grow.

Scala - Ever since I built my first Scala project, every time I work in another language I wish it were Scala. Static (but aggressively inferred) typing, first-class functions and closures, easy currying, terse syntax. Performance is even quite good (on well-designed programs), since it runs on the JVM rather than some homegrown runtime. Plus that means easy compatibility with any Java library.

dart
It has what you need from javascript with classes, interfaces, and variable types.

Adding to my list of favourites:

• AGG (Anti-Grain Geometry)
  
• glAArg
  
• GPC (General Polygon Clipper)
  
• Wild Magic (by www.geometrictools.com)
  

Lua with Love 2D

dart
It has what you need from javascript with classes, interfaces, and variable types.

I looked into dart shortly after it was announced, and the project was in a sorry state. It didn't compile for windows, the documentation was missing or bad, most links 404ed, etc...

In other words, a pretty typical google developer project...

Has the organization improved since, or is it yet another half supported Google release?