Here you will find links on aspects of singing not related to the other two categories such as the recording process, finding sheet music and backtracks, and more!
For those of you into recording, mixing, and production, this forum is a great resource. There are many professional sound engineers on the site that would be able to give hints on various aspects about the recording and mixing process, in addition to the industry in general.
A large selection of sheet music.
A digital sheet music store. All of their sheet music can be directly downloaded to your computer. No need to wait for shipping!
An online store for backing tracks. They even allow you to edit and/or transpose your songs so that they custom fit any needs you might have.
Web site of vocal scientist Ingo Titze. There are many articles on aspects of technique, getting into the hows and whys, in addition to some information on the anatomy of the voice.