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.)
» Dark Water: The Series Bible
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.).
» World Book [PDF (no images)]
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.)
» Map of Mer: very large (1.1MB) | medium (412KB) | small (216KB)
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. I currently have pages 33-57. HUGE thanks to Jacob for mailing these to me.