Okay, well, I'll help fill in some of the blanks since the OP is really out of date. Most of those forums have been deleted.
I can tell you that one of the categories is called Archive - v8 and includes the forums Archive v8 - Add-Ons, Archive v8 - Maps, and Archive v8 - Faces, Decals, Prints. Shortly after v9 was released these forums were publicly accessible. v9 introduced a new add-on package format based on zip files. Before that add-ons were just random files and folders thrown in the add-ons folder. The boards were archived to give add-on owners an opportunity to convert their add-ons, then they were closed down after a few months. You can see some of those forums up in the main post. They've been merged, deleted, and renamed since then.
In the Files category, there is a forum called the Fail Bin. The Fail Bin contains add-ons in it that were deemed so horrible that they had to be hidden away for the safety of the community.
In the Blockland Forums category, there is a forum called Beta Discussion. This forum was initially used to discuss changes and features to the game before it was released. After Blockland was released, it continued to be used to test pre-release versions of new Blockland versions. The last version of Blockland we tested was v19. Now there's a constant update system and we test individual features on private servers, and news is posted to the public development board, so the forum has fallen into disuse. The last post in that forum was from December, 2011.
In the Off Topic thread there's a forum that mostly contains non-Blockland related projects produced by Badspot and the beta testers. You might see some of them if you follow PAX this year. The name of this forum is Cool People but this is in fact a misnomer, so don't feel bad that you can't view it even if you are a cool person. According to an official poll administered by kompressor, only 37.5% of the people who can access this board are in fact cool.
There is one more category and 2 more forums that I can't view. If had to guess, I would assume that it's a test category, and it probably contains a test forum, possibly for the RTB mod bot. I assume that the other forum is probably full of pictures of sharks skateboarding over an exploding volcano.
