Wikimedia
Wikipedia: Operetta
This Wikipedia site offers excellent information on the artform of the operetta, including its history, important contributors, and hyperlinked terms.
Other
The Aria Database
The searchable Aria Database features over 1,200 arias from 170 operas by 60 composers. It includes lyrics, translations, synopses of operas, and MIDI files.
Other
J.s. Bach/texts of the Complete Vocal Works
A complete listing of the vocal works with their librettos. Very comprhensive. Click on BWV texts to view words.