Most Opera composers can be found in this site or, more generally, might be present in this Classical composers database .
If you miss someone, however, you cannot avoid a consultation of the Underrated masterpieces.
For quick news on some composers, perhaps you would be quite satisfied just in entering the Index of composers
If you would like to understand also what the hell they say during the play, then go to e-libretto
A lot about opera can be found in Mario Biondi's site
A celebrate soloist that has composed operas as well: Giovanni Bottesini
A basic Verdi's page
And, strictly in alphabetical order, a little collection of Italian opera composers:
Paolo Serrao (in Italian)
Emilio Usiglio (in Italian)
Discografia - La casa della musica
And if you miss someone...go to Dizionario dei compositori Italiani or to Répertoire international de la presse musicale
To find an unusual CD: Opera rara
A curiosity: Manifesto della musica futurista
More on Opera, albeit in Italian, can be found here: Operaclick
And the rest is coming soon...