There are many great resources on the web. This is just the tip of the iceberg. If you find a great site that you would like to share, please add it to the comment section below, and I will try to add it to this site.
The Lieder and Song Texts Page – This is an archive to texts and translations of lieder and art song. It is very helpful.
Art Song Central – This is a fantastic archive of free, public domain sheet music. Packed with standard vocal repertoire.
Classical MIDI with Words – This site contains MIDI files of many art songs and arias. It can be a useful practice tool, if you can put up with the sound that MIDI makes.
The Aria Database – This site contains information, translations and MIDI files of many opera arias. It is a fabulous site.
MatchPitch – This is one of the coolest things I have seen. You need a microphone and a PC to make this work (I have not found a Mac version unfortunately). The software allows you to click a note on a graphic of a piano. It plays the pitch and displays a colored line to go with it. Then you sing and try to match that pitch. When you have matched it, your colored line will superimpose over the existing pitch line. Very, very cool. This is particularly wonderful for people with pitch matching problems. You can download the demo (good for 10 days) for free. If you like it, it is well worth registering and paying the fee.
Vocal Lab – This software sounds promising. I have tested it. Here is the description from the website:” Vocal Lab helps you train to sing on key. It listens while you sing and displays the pitch of your voice on a graph, in real time.” Mac users only. Shareware.