Another Moscow site.
The Bolshoi Theatre.
We suppose there's nothing else to add about The Bolshoi Theatre. It's one of the biggest opera and ballet theatres in Russia and one of the most significant and renowned theatres in the world. As the Russian ballet is definetely a must-see, The Bolshoi Theatre is a must-visit (or, at least, a should-visit :) ).
This is the official website in English. There you can find all the latest info on the theatre (including ticket info and its address, certainly).