Spotify A free music streaming service with a large classical catalog. This is a must have for all aspiring musicians. Unfortunately, you must have a Facebook account.
IMSLP.org This is a great place to find any music that is in the public domain. Mostly, this includes works before 1923. For violinists, this means access to free studies, like Sevcik and DeBeriot, but also thousands pieces in the rep, including works by Bach, Biber, Mozart, Wieniawski. The real treat is finding obscure works by people like Achron or Ovide Musin.
British Library Sound Collection - British Library Sounds presents 50,000 recordings and their associated documentation from the Library’s extensive collections of unique sound recordings which come from all over the world and cover the entire range of recorded sound: music, drama and literature, oral history, wildlife and environmental sounds.
Sheet Music, Strings, and Accessories, at Shar Music.com - Free Shipping specials on strings.
Sheetmusicplus.com - for harder-to-find sheet music.
Free Printable Staff Paper, for writing your own music.
Muse Score - A well designed, free notation software.
Trial Versions of Finale, music notation software.
Music Theory.net - A great music theory resources, appropriate for high schoolers and adults.
Violinist.com - Discussion boards, etc. for professionals and amateurs.
reddit.com/r/violinist - a growing violin-based online community