As the name suggests Audiotuts is a series of tutorials designed to help with your audio. There are tuts on mixing and mastering as well as lists such as 11 Totally Free VST Synthesizers That Sound Great.
AudioJungle is a similar site, but instead of tutorials it has articles meant to help you out with your music, and also answer questions you may have asked yourself, like what is the role of an egineer. Check out their list of apps and sites to train your ears, and also 8 Free, Cross-Platform Apps for Musicians, these are bound to be useful. They're also giving away an Mbox 2 Pro at the momment too.