Internet is an extraordinary source of information
on singing.
Here are some links with sites I particularly enjoyed.
the flags indicate the language of the sites.
Forum
Opera 
Music
& Opera Club 
The detailed programmes of more than 500 venues, orchestras and festivals
in 35 countries. Search by destinations, performers, works.
Book your tickets around the world with the Music & Opera Club
Nicole
Scotto di CarloFascinating !
If you want to find out everything about the singing voice, from the technique and physiological point of view, this site is a mine of information in order to understand how the classical singer produces sound.
I particularly recommend the articles for the non-specialist.
Fondation
Lotte Lehmann Essential !
I am a consultant for this American foundation which is trying to promote and popularise ‘the art of Lieder and Songs” in the United States.