AllMusic claims to be the "world's largest" and "most comprehensive" entertainment database for music.  It offers expert reviews, biographies, ratings, images, and titles.  You can search by artist(s), album(s), song(s), style(s), or by label(s).

Berklee Shares: Free Music Lessons

The Berklee College of Music offers free music classes in MP3, PDF, and Quicktime format.

Essentials of Music

Essentials of Music was created in cooperation with W.W. Norton & Company and Sony Classical.  It is a concise guide done by 200 experts from ESSENTIAL CLASSICS.  Explore the "Eras" section for overviews of the main periods of music history (Middle Ages, Renaissance, Baroque, Classical, Romantic, and 20th Century).  "Composers" section offers brief biographies of nearly 70 composers.  The "Gossary" section explains over 2000 musical terms with musical examples.

FindSounds

FindSounds searches the web for sound effects and sample sounds.

Instrument Encyclopedia

The Instrument Encyclopedia is a collection of information on musical instruments from different cultures.  Browse the collection by instrument types (percussion, string, wind and electronic) or browse by geographical origin.  Generally you will receive background information, a picture, and further reference sources, but it may vary.

JamBase

According to JamBase site they are "the leading provider of tour dates, information, reviews, articles and links for thousands of improvisational bands worldwide, JamBase has evolved into the community hub for our ever-growing scene of music. Covering every derivative of rock, jazz, funk, trancefusion, bluegrass and live electronica, JamBase strives to break down barriers between musical genres and provide a daily resource for musicians and fans alike."

MacMusic

MacMusic is a music directory of over 2000 sites which can be accessed by Macintosh computer users.  It provides the latest music news concerning Mac users.

Mojam

Mojam is a connection to help locate music concerts in the United States.  You can search by artist, by venue, or by city.

Museum of Musical Instruments

The Museum of Musical Instruments offers a virtual collection of guitars, mandolins, and musicians of the last two centuries, with photographs, audio files, and exhibits.

Blank Sheet Music

A repository of blank sheet music that you can create yourself.

Worldwide Internet Music Resources

The Worldwide Internet Music Resources site is done by the staff of William and Gayle Cook Music Library at Indiana University School of Music.  They have complied an exhaustive directory of musical resources on topics such as:   classical performers, groups and ensembles, genres and types of music, research and study and much more.