D1 Wiki Porky's Wiki dedicated and devoted to the development of the D1 concept which you can read more about at Porky's Expanse.
Dagonbytes -- this site seems to have gone away. You can get your H. P. Lovecraft from the always reliable and excellent Lovecraft Archive instead.
David Mamet's infamous/notorious Master-Class Memo to the writing staff of The Unit. Some excellent insights in there for any writer or GM.
Dictionary of Drygoods An encyclopedia reference covering all known forms of fabric and a few other fun things.
Dieselpunks A site devoted to alternative forms of Steampunk and those genres that have grown out of it that aren't steam-oriented. Dime Novels & Penny Dreadfuls A lot of good information on these predecessors of the modern revival of Serial Fiction. Dennis Wheatley An extensive essay at The Weird Review--one of the best essays/reviews of the guy's work you're likely to find on the internet.
Doctor Grordbort Fun Stuff. Where Science Meets Violence. Check it out. Donjon One of those newfangled games trying to go all Old School in a fresh new way, whatever the hell that means...