AA-lib is a low level gfx library just as many other libraries are. The main difference is that AA-lib does not require graphics device. In fact, there is no graphical output possible. AA-lib replaces those old-fashioned output methods with a powerful ASCII art renderer. The API is designed to be similar to other graphics libraries.
Agar is a toolkit for graphical applications. It features a modern GUI system with an unobtrusive API. Agar can use different graphics interfaces, such as SDL and OpenGL, and it has been ported to numerous platforms, including game consoles and embedded systems. Agar is implemented in C, but can be used from other languages.
Allegro is a multi-platform game library for C/C++ developers that provides many functions for graphics, sounds, player input (keyboard, mouse, and joystick), and timers. It also provides fixed and floating point mathematical functions, 3D functions, file management functions, compressed datafile, and a GUI.