Sources of information that I have found very useful for research, scores, lyrics, librettos, audio files - the list goes on...

The Lied and Art Songs Text Page - an invaluable resource for tracking down the lyrics for Lieder and with translations into English, French, Italian, Spanish the list goes on...
Set up by Emily Ezust and given server space by the REC Music Foundation, this site is unfunded - so if you find it useful, please try to donate something to Emily.

Schubertline - a wonderful resource for those whose sight-reading skills need a little help when learning a new song. Scores of music are rendered up by Scorch (you can download the software for free) and allow you to playback scores of the past masters : Schubert, Mozart, Handel, Debussy et al. The scores are also printable for a fee.

The ChoralWiki - The Choral Public Domain LibraryBegun in December 1998, CPDL is a free resource where you can find scores, texts, translations, and information about composers.