This site is really a helpful in understanding the Linux OS and is a good start if you are planning on coding anything for the OS.
Yeah thats right........I said It!
Ultimately everyone will have their own opinion--this is mine.
another really good site is www.google.com/linux
(for a site with a list of links that is)
Might as well add these to the list
Another helpful site, especially if you have linux installed or a live cd to play around with: