This is a completely biased statement. First off it's impossible to say that SDL is the 'most popular' 2D library out there when there are about 2 books I even know of about SDL and hundreds and hundreds for using 2D in 3D with OGL or D3D. I would even say that Allegro beats SDL in popularity.
There are many, many, graphics libraries to choose from. One of the more popular 2D ones is the SDL. The SDL is a "multimedia toolkit" with support for audio, video, cdrom, mouse, joystick, and timer support. Another is Allegro.
Most people reading this page don't need to know about a 3D graphics library, but anyhow. The most popular 3D graphics libraries are OpenGL and DirectX. 3D graphics is many, many times more difficult than 2D graphics