Please list (and date) the different Eras from Grand Opera to whatever
is in vogue at this very minute.

I have not found any site which lists pop musical genres by era,
including opera and extending to today's music. It was an interesting
search, and I hope that some of what I came across may be of use to
you and to other Researchers.

Wikipedia offers a nice list of popular music by genre, and also a
timeline of musical trends:

Wikipedia
http://www.wikipedia.org/wiki/Popular_music

Wikipedia
http://www.wikipedia.org/wiki/Timeline_of_trends_in_music_(1951-present)

There's some useful info on the history of American pop music here:

Kingwood College Library
http://www.nhmccd.edu/contracts/lrc/kc/music-1.html

Kingwood College Library
http://www.nhmccd.edu/contracts/lrc/kc/music-2.html

Kingwood College Library
http://www.nhmccd.edu/contracts/lrc/kc/music-3.html

Here's a history of country music, by era:

Roughstock
http://www.roughstock.com/history/

A nice big bands/jazz timeline:

Big Bands Database
http://64.33.34.112/.WWW/timeline.html

A disco timeline:

Jahsonic
http://www.jahsonic.com/Disco.html

Here's a pop music timeline by era, including these categories, with a
brief discussion of each:

1900 & 1910 Ragtime
1920 Scat
1930 Big Band
1940 Bebop Jazz
1950 Rock N Roll 
1960 Rock Music
1970 Disco
1980 Rap
1990 Alternative

Alison Farrand's Website 
http://users.rcn.com/farrands/teaching.html

~Pink

Many years ago I had a book about the history of rock and roll that
was published by Rolling Stone Magazine. I can't recall the title, but
the book contained a detailed time-line of musical genres, including
rock and roll (up through the 1980's, I believe.) Maybe someone will
be able to find that book for you.