Links

Teoria.com
This site has a broad collection tools for learning music theory and analysis. The learning FLASH objects on the site are divided into three categories: Tutorials, Exercises and Reference.
Download MiniTuner at the Jorge Guillen site
This handy program will let you tune your musical instrument by using a microphone through your PC's sound card. MiniTuner automatically detects the pitch of the input signal and displays the corresponding note and its deviation from the targeted note.
Metropolitan Opera Guild
Scroll down to find opera synopses, biographies for singers, conductors and composers, production photos, sound clips, background information, and more, all translated into several languages.
A very good and free Music Dictionary. It also has a great page of musical symbols. Another alternative is the Virginia Tech Multimedia, with term pronunciations and music examples in Flash.
For a more complete resource, you may want to try the wonderful Grove Music Dictionary Online. It is the mother of music dictionaries out there (It is not free, however you may sign up for a free subscription period).

Check out John Henry Sheridan's page.
Los Chantas, a great tango ensemble based in NYC. Check out their calendar and do yourself a favor: go listen to them. They very often play in the Boston area.
Vox Novus, created by Robert Voisey for the promotion of contemporary composers and their music via concert performances, recordings, publishing, and publicity on the Internet.

WeddingMusicUSA is an association of professional wedding entertainers located throughout the USA. The site also contains articles on planning the music for your wedding ceremony and much more.
WedAlert. This wedding planner website has, amongst plenty of other useful information, a great library of audio files with suggestions of music for your wedding.
Brian Ahronian is not simply a Disk Jockey/MC. He will also sing at your wedding for a live music experience without the pricetag.

Guitar Specific

Delcamp Forums
Probably the best resource website for classical guitarists. Other than the more than 2,000 pages of classical guitar sheet music, and 3, 000-plus MP3s, this site has a great forum where guitarists, novice and experienced, share their ideas, technique, and love for music.
Frets.com
"The Online guitar owner's manual". A great resource in case you need to discover more about the mechanics of your instrument.

At Teachlist you can find information about other local music teachers in the area.
Brazilian Endowment for the Arts, site of the Brazilian-American Library in NYC and non-profit institution geared towards development of the Brazilian Cultural expression in the USA.
A nice blank chord chart in PDF. Download it here.