Violin Masterclass - Famed pedagogue, Kurt Sassmannshaus, hosts a comprehensive library of video tutorials and performances, along with practice guides and repertoire lists.
Violinist.com - A treasure-trove of everything violin-related! Most famed for its discussion forum, it also has links to interviews, blogs, and online retailers.
Viola.com - Known for its email group, The Viola List, Viola.com is devoted to resources and information only applicable to violists.
American Viola Society - Mostly geared towards professionals, the AVS is a great place to learn about competitions, studios, and read past issues of the Journal of the American Viola Society.
Cello Bello - Created by New England Conservatory professor, Paul Katz, Cello Bello features blogs by famous cellists, video masterclasses, streamed online events, and information on competitions.
Internet Cello Society - Much like Violinist.com, but for cellists! ICS features an active discussion forum, as well as tips, information on competitions, and links to online stores.
Music Theory Resources
Music Theory.net - Animated music theory lessons that are supported by apps for your iPhone, iPad, or iPod Touch. A clean interface and simple language makes learning theory enjoyable!
Teoria: Music Theory Web - Music theory lessons supported by video and audio examples. The exercises are more customizable making them perfect for more advanced students.
The Violin Shop - When repairs are needed, San Diegans take their instruments to Kevin Smith. They also have a large inventory of high-quality instruments and host several teaching studios.
San Diego Violins - Formerly Hammond Ashley Violins, SDV boasts a large fleet of high-quality rental instruments. They also specialize in bow rehairing.
SHAR Music - SHAR is the #1 online retailer of stringed instruments and accessories. They carry products that cater towards professionals and advancing students at great prices.
Southwest Strings - SWS doesn't carry the more advanced instruments and accessories that SHAR does, but they do have a "Best Price Promise" ensuring you never overpay for the things you need.
American String Teachers' Association - ASTA provides educational resources to string teachers as well as opportunities for their students to excel - like the Certificate Achievement Program.
Music Teachers' Association of California - MTAC offers the famous Certificate of Merit program as well as performance and composition opportunities for all students of its members.