After one week of private beta started April 27, the new music.stackexchange.com has been launched.
The goal is to have a place where serious questions about music (instrumental technique, music theory, performance, practice, health, maintenance, instrument making, composition …) can be asked and answered by musicians. This is free, anyone can join, even anonymously.
Take some time to have a look. If you already know English.stackexchange.com, stackoverflow, math.stackexchange.com, etc. the principle is the same and you can share some reputation between the sites.
EDIT: Here is a snapshot of the current site (design is not final but rather common to all beta stackexchange sites)
EDIT (2001/05/28): the previous Guitars stackexchange site has been merged into this site, featuring now several hundreds additional questions about guitar technique, practice and performance.
You can have a global look at music.SE current statistics here