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For me,
STB libraries, including minimal image loader, ogg/vorbis decoder, and truetype font rendering
SDL and GLFW, it's sometimes hard to decide between the two - SDL has sound support and more supported platforms, while GLFW is compact and minimally obtrusive
Recast & Detour as well, I hope to get to using this some day, but have had no time yet
AngelScript, very convenient scripting integration with C++
I'm also following other newer, less known open source game engines such as LightFeather, SoftPixel, PixelLight and hmrEngine.