These libraries can extend the core SDL functionality.
Loads images as SDL surfaces and textures, and supports the following formats: BMP, GIF, JPEG, LBM, PCX, PNG, PNM, SVG, TGA, TIFF, WEBP, XCF, XPM, XV.
SDL_mixer is a sample multi-channel audio mixer library. It supports any number of simultaneously playing channels of 16 bit stereo audio, plus a single channel of music, mixed by the popular FLAC, MikMod MOD, Timidity MIDI, Ogg Vorbis, and SMPEG MP3 libraries.
A library which allows you to use TrueType fonts in your SDL applications. It comes with an example program "showfont" which displays an example string for a given TrueType font file.
A small cross-platform networking library, with a sample chat client and server application. The chat client uses the GUIlib GUI framework library.
A library which allows you to display Rich Text Format (RTF) documents in your SDL applications. It comes with an example program "showrtf" which displays an RTF file using an SDL_ttf font engine.
SDL2, through SDL_RenderGeometry, can be used as a back-end with other GUI/Animation libraries:
RmlUI (fork of libRocket):