As mentioned in rand al'thor's answer, [Chakoteya][1] is very useful, but if you're interested in the original scripts (with scene directions and such) rather than transcripts, you can find all the scripts for the movies, TNG and DS9 (but few or none for other Trek series) at [st-minutiea.com][2].


  [1]: http://www.chakoteya.net
  [2]: http://www.st-minutiae.com/resources/scripts/