Extensions

Vertex Buffer Object
Frame Buffer Object
Non squared, non power of two textures
Vertex shaders and Pixel shaders
Multitexturing

Actually go here

http://oss.sgi.com/projects/ogl-sample/registry/

and read or pick out the ones you want to use. I suggest you look into GLEE or GLEW extension loader libraries. If you are only adding a few functions doing it yourself is fine, but things like GLSL, FBO, VBO, you start adding in all their functions and it gets to be a lot. It nice to just call a few functions to setup the extensions and then just code with them like any other functions... Just my two cents.