Simple Direct Media Layer for which I use in R_Engine. You can also find links for SDL_Mixer and SDL_Image (both in the projects page) both of which I use in R_Engine.
For all game development issues and needs.
A C++ programmer's best friend and ally. I used boost in R_Engine for various things.
Great lightweight gui library for c++. Perfect for making custom gui components. I used this for Icha-Icha Rescue.