In no particular order...

Radifiy yourself!
I like Rad. Just do.

W3Schools
A good place to start. Their instructions are very clear, easy to read, and they have a lot of examples. There are also many friendly references there. Super highly recommended for those of you that want to code the web correctly.

The Happy Hacker
A fine site, indeed. Carolyn P. Meinel (a sewpirkewl lady) runs/maintains/puts a lot into this site. If you are just moderately interested in the subframe of a computer, I highly recommend you start here. ;)

Lazy Foo'
An excellent resource for someone looking to get into programming with the SDL (Simple DirectMedia Layer) library. Some other programming advice is there, to boot.

D
Home of the D programming language, created by Walter Bright. I recommend it even though I barely understand it. :)

Derelict
An excellent collection of libraries and extensions for D: SDL, OpenGL, OpenAL, ... A must have, really. :)

D Porting
This is what I used before I used Derelict. It hasn't been updated in a while.