Most of these dedicated fans who write fanfiction probably already have, but I figured I'd share them anyway.
Daystrom Institute Technical Library. A veritable potpourri of useful links and factoids about the Trek verse. A lot of this stuff is purely fanon or based on noncanon stuff, but it can certainly be useful from time to time. Fair warning, they have a positively annoying Christmas theme right now that is beyond my capabilities to turn off. The thing I found most useful was the warp speed calculator (for when you want to sound at least vaguely accurate instead of simply Speed of Plot.)
Ex Astris Scientia - In a similar vein but (IMO) more useful and slightly better put together site, this has a whole host of neat things on it, including a uniform section that has all of the Starfleet uniforms. Like the DITL, there's some stuff of questionable canonicity but overall, I found this site to be as useful as Memory Alpha. Which leads me to...
Memory Alpha. This is the place to go for canon Trek information. Very, very useful, and I'm surprised there isn't already a link for it here. Unless I missed it which is totally believable.
Of middling use is this nifty article on Explosive Decompression and Vacuum Exposure. To nobody's surprise, Hollywood generally gets it wrong.
Internet Encyclopedia of Notable Stars is just interesting as all get out to me, but hey, I'm a geek. Equally interesting is The Internet Stellar Database
Of much use is the Romulan-English Dictionary. Yeah, it uses purely non-canon (albeit official) source, but it is still pretty useful. Similarly, the Rihannsu Encylopedia has some nifty and useful stuff in it.
And, of course, there's my site which my roommate made purty...
