It uses DocFetcher, which isn't very flashy, but it does what it says it will do (works for Linux and Win from the readme.txt, using Java. No clue about Mac).
It has lots of things that many folks probably already have/use as reference material, but also...An ENORMOUS collection of searchable NWN reference material in one place!
What is the NWN Omnibus?
It is an attempt to gather the majority of NWN reference material together in one place and allow very quick keyword searching against all of the documents at once. It includes reference documents as well as forum threads.
If there is something better, I'd love to hear about it.BioWare Forums - Offline archives of the following forums from the old BioWare website, each thread in its own .HTML file. The following full forum archives from the old BioWare site are included in this release: NWN Custom Content (ARCHIVE), BioWare News Discussion, Custom Content, For Developers, Scripting, Server Admin & Toolset. Also Stickied threads for every forum were included.
*edit* Upping the "Results Per Page" from 50 to 500 or more in the preferences helps a bunch if you don't like flipping through pages of results. Also, some source material looks better using the "embedded HTML viewer" (top right button), and some doesn't, so experiment with it.
*edit 2* Link updated to point to the new vault replacement version