SDL stands for Simple DirectMedia Layer and it was developed by Loki Games (maintained by Ryan AFAIK) as an API layer to easily port/develop games by providing interfaces to networking, audio, video, etc.
While SDL is not DirectX, it's a close equivalent and it did make Loki's task of porting DX games relatively more straight forward than without. At least that was the theory, hehe.
I reckon some commercial Linux games use this (all Loki ones, I think) as well as games like Neverwinter Nights.
SDL's video section, I think, kind of wraps to OpenGL. It's been a while since I've last handled SDL though so I could be wrong.
I'm sure Svartalf can provide more accurate info. I feel old already. hehehe.