Thread to collect most rich and useful datebases with librettos in web.
I generally start this thread because I need one, but I guess I have one to recommend:
http://www.rwagner.net/ - Wagner librettos for all operas, original and english translation combined and posted side by side so you can not only read and understand texts of arias and stuff but also put translated words correctly to what you hear in German or just support your reading with translation if you have good German but are not sure about some things
Caveat: this site always has the original language and the side by side translation in Spanish only.
It's fabulous for those who speak Spanish, but not as useful for those who don't.
It is more complete than most.
"J'ai dit qu'il ne suffisait pas d'entendre la musique, mais qu'il fallait encore la voir" (Stravinsky)
What's wrong love me strong ding dong I can't find anything, how can they expect me to like Donizetti when one can't find libretto to Anna Bolena and how can I enjoy thrilling duet of Callas/Corelli in La Vestale by Spontini if even plot synopsis in the web is so short, or Il Pirata, actually I'm starting to think that there are absolutely no librettos in the web, let me see