This morning when trying to find an English translation of a passage from Nabucco, I stumbled upon Opera Guide, a site I hadn't yet encountered. Available in English and German as part of Kernkonzepte's Impresario site, Opera Guide contains a huge listing of synopses and libretti (many of them translated into English) arranged alphabetically by composer. Opera Online is the paid tier of the Opera Guide service containing sheet music, MIDI files, and parallel texts payable via PayPal donation in order to keep the site entirely non-commercial.
You can find a permanent link to Opera Guide on the off-site resouces section of the sidebar.
Link to Opera Guide
Link to Opera Online
Link to Impresario portal
Link to KernKonzepte