SDL

Adgali
A game library useful for 2D games programmation. It uses the SDL library for the bitmap, sound and events which provides accelerated and portable interfaces for the video and sound hardware. This library tends to provide useful functions for bitmap and sprite display, game GUI, sound events and music, generic AI engine, network management and video streaming.
http://adgali.apiou.org/

Documentation Project
Includes guide, tutorial, and reference with examples for the library as a whole and the individual methods.
http://www.libsdl.org/cgi/docwiki.cgi/

Drac Card Game Library
A portable C++ card game library that uses SDL. [Open source, LGPL]
http://drac-cardlib.sourceforge.net/

EZ2D
A 2D game engine. It uses SDL as its main API (sorry DirectX users), but very little knowledge of SDL is actually required.
http://ez2d.sourceforge.net/

GFX with SDL
A SDL page for beginner's. Tutorials on how to: initialize SDL, load and display images, and how to load sprites.
http://cone3d.gamedev.net/cgi-bin/index.pl?page=tutorials/gfxsdl/index

Kyra
A simple, fully featured, industrial strength sprite engine written in C++. (Open Source / LGPL)
http://www.grinninglizard.com/kyra/

LibKSD
A cross-platform game framework written in C++ on top of SDL. [Open Source, LGPL]
http://libksd.sourceforge.net/

Roger's SDL Tutorials
Learn how to use SDL_Mixer, and SDL_RWops.
http://kekkai.org/roger/sdl/

SDL Tutorial Wiki
SDL game programming tutorials, source code and links.
http://gpwiki.org/index.php/C:SDL_tutorials

SDL_gui
A library written in C++ for displaying and controlling user interface elements in an SDL application.
http://rhk.dataslab.com/SDL_gui/

SDLMappy
Library that uses 2D tiles based maps created by Mappy. It allows layer transparency, animations, and the creation of up to eight different layers.
http://membres.lycos.fr/edorul/SDLMappye.htm

SDLSprite
This C++ class contains the data and functions required to display animated sprites from a BMP file.
http://membres.lycos.fr/edorul/Othere.htm