Strings Magazine, Strad Magazine, Fiddler Magazine feature articles on performers, teachers, instruments and makers, product reviews, summer festival and music school directories, and news in the field.
Violinist.com features interviews with leading performers, user-contributed blog posts, and forums. Viola.com hosts an e-mail list serv which usually features thoughtful and historically rich discussion on the viola, viola repertoire, and performers. Maestronet.com is an online forum that tends to focus more on instrument making.
BulletProofMusician.com blog by Noa Kageyama addresses motivation, overcoming performance anxiety, and other constructive psychological skills of successful performers.
MusicialChairs.info hosts information about upcoming auditions and instruments for sale worldwide.
In addition to your local music store, it can be helpful to compare prices for strings and other musical equipment at Amazon.com, Shar Music, Southwest Strings, and Musician’s Friend. There are many other fine online vendors, but these are the websites I tend to check the most often for competitive prices.