If I think about a website that I use so much in relation with my career, two pages come to my mind.

Years ago when I was studying at an academy  a singing teacher showed me the website 

www.kareol.es. It´s a website where someone can find translations of many operas and other types of musical works like songs or cantatas.  Furthermore, it´s very simple to surf the web   because you have fourth options on the principal page, you select a searching method and then the works appears in alphabetic order. For me it´s very useful to translate scores and read the full libretto of opera.

Another website is www.imslp.org and it´s my favorite. Basically as the page says, it shares the world´s public domain music. You can find a huge variety of composers´ scores from many countries and many periods in the history of music, of course, different languages and instruments as well.  On this website if I am looking a specific score, I only put the name on the imsl search and appears.  I use this page so many times during the week, because I like to find unknown scores to incorporate them in my repertoire.