DITL: Daystrom Institute Technical Library: http://www.ditl.org/
-Under the "Calculators" button has stardate conversion, and time/warp speed/distance calculators! You don't have to guess anymore!
Advanced Starship Design Bureau: http://techspecs.acalltoduty.com/federation.html
-Has VERY detail specs on many starships
Jeff Russell's Starship Dimensions: http://www.merzo.net/
-Vessel size comparison from many different sci-fi shows
Ex Astris Scientia: http://www.ex-astris-scientia.org/index.htm
-Ship size charts, episode guide, etc.
...just to name a few.