Royal College of Organists http://www.rco.org.uk/
Choral Public Domain Library http://www.cpdl.org/
French Organists http://www.nzorgan.com/events/Jacquesback/frenchorganists.htm
Music Teachers http://www.musicteachers.co.uk/
The Study of Fugue http://jan.ucc.nau.edu/~tas3/simplefuguesaof.html
St Giles Organ School http://www.organschool.com/
JS Bach Home Page http://www.jsbach.org/
Organs and Organ Music http://www.orgel.com/links-e.html

 

If you are studying organ music,  the links opposite contain a lot of useful information