There are wonderful resources on the Web for deepening one's knowledge of the world of singing.  Here are a few of my favorites: 

For the Art Song

The Lied, Art Song, and Choral Texts Archive was begun by Emily Ezust in 1995 as a place to locate texts set to music in a wide variety of languages by an astounding number of composers.  The site describes itself as "a collection of texts used in 104,394 Lieder and other classical art songs (Kunstlieder, mélodies, canzoni, canciones, liederen, canções, sånger, laulua, písn?, piosenki, etc.) as well as in many choral works and other types of classical vocal pieces. The archive indexes 65,496 texts with 13,211 translations to English, French, Italian, Dutch, German, Spanish, Portuguese, Chinese, and others. The website has frequent updates." If you ever wanted to find variant settings of famous poems, like "Kennst du das Land" (Mignon's Lied, by Goethe), here is where you might start.

Song Literature (Scores) Online — a prototype at the William and Gayle Cook Music Library at Indiana University — provides free online access to a wide variety of Lieder and Art Song.

For Opera

Opera Glass is an excellent meta-site for links to Opera Resources worldwide, hosted by Stanford University. It includes links to Opera Composers, Opera Librettists, and Operas. Most operas listed provide librettos as well as synopses.

Online Opera Scores — a prototype at the William and Gayle Cook Music Library at Indiana University — provides free online access variously to full and/or piano-vocal scores of the major opera literature.

Operabase is a good way to research the people who make operas happen, as well as the operas that are happening. A powerful search tool allows you to search the details of over 38,000 opera performances since August 2010. Artists and managers are indexed, too.

Seatgeek – Learn About Opera Theater is a great information resource about opera aimed primarily at younger audiences and performers. Besides providing very easy-to-understand definitions and descriptions, it includes a wealth of links to many related opera topics.

For the Voice

The Voice Foundation is the world's oldest and leading organization dedicated to voice medicine, science, and education.

TheSingersResource.com has a "Vocal Health" page with basic advice for care of the voice,

How Your Voice Works is a site created by the NYU Voice Center, with basic information on the vocal cords and breathing.  It has links to educational photos and videos, but is more clinical than most students will need.