Daystrom Institute Technical Library
DITL has a ton of information on anything related to Trek, from starships to clothing.
In Klingon/English.
Klingon version of Wikipedia. (In Klingon)
Memory Alpha is a collaborative project to create the most definitive, accurate, and accessible encyclopedia and reference for everything related to Star Trek.
ST-I is a great site for any information about starships. It also has a small Federation Technology section.