The SDL_gfx library (http://www.ferzkopp.net/joomla/software-mainmenu-14/4-ferzkopps-linux-software/19-sdlgfx) evolved out of the SDL_gfxPrimitives code which provided basic drawing routines such as lines, circles or polygons and SDL_rotozoom which implemented a interpolating rotozoomer for SDL surfaces.
The current components of the SDL_gfx library are:
Graphic Primitives (SDL_gfxPrimitves.h)
Rotozoomer (SDL_rotozoom.h)
Framerate control (SDL_framerate.h)
MMX image filters (SDL_imageFilter.h)
Custom Blit functions (SDL_gfxBlitFunc.h)