Official, and very rare, content/documentation from Hanna-Barbera or elsewhere that pertains to and elaborates on the series. (This may expand to include rarely-seen content from The Saga Begins and the Dark Water miniseries, if I'm able to provide content from those.)
This is the real thing. This was originally a typed document I received as an "extra" when I won the RPG module off eBay (the seller happened to be the author, who still had his copy of HB's PoDW documentation and sent it to me), and I've typed it up and converted it to HTML format--this is a 50-plus-page document Hanna-Barbera put together prior to the release of the show. Note that some of the descriptions and names are slightly different (episode 2's title, some of the episode plot intricacies, etc.).
The 62-page book accompanying the official RPG module released by Mindgames. This includes a history of Mer, information on several main species, characters, weapons, vessels, different types of magic and magical artifacts, locations around Mer, and the actual gaming system. (A note on the navigation: the dropdown menu on the first page will open the page of your choice in a new window. There are also prev/next links on each page if you want to go through page-by-page.)
NOTE (July 2009): Many of you have pointed out that this is incomplete. I am working on typing up the parts of the World Book I omitted (not being a gamer, I figured initially that people could just make up their own stats for the various characters--now that I know better, I do apologize sincerely for that). I am also working on scanning the 100 character sheets also in the RPG module, and capturing the text via my scanner's built-in OCR so that I can produce a PDF. Here are the character sheets I've scanned so far.
This 8-page fold-out color map accompanies the RPG module. (I had to scan it in in pieces and digitally piece it together, so if some areas are a bit blurry, I apologize.) I've included 3 different sizes, all varying in dimensions and in quality, depending on whether you want a general view of Mer or details of what goes where.
Schematics of pretty much the entire Maelstrom (9 separate images), as well as of scout ships and other peripherals.
Official HB guidelines on the appearance, colors, and so on of the major characters, ships, and locations from the show. These are complete as of July 3, 2009! (Huge, huge thanks to Jacob for initially mailing me half the files, and then hand-delivering the rest in person!)
I'm really not sure where else these fall, but a few people have e-mailed me advertisements for the show that were published in various places, and for now they're getting their own section.
An article from a back issue of the Washington Post, found by Chris through his school's database search engines. It's about David Kirschner in general, with an emphasis on Dark Water, and was written around the initial premiere of the show. There's some very interesting and telling content in there, and I'd highly suggest you check this out!