Chazzanut Online
A very comprehensive website about chazzanut, from Irwin Oppenheim in the Netherlands. This site has many features, including biographical materials on major cantors, notated nusach, a discography, a list of online archives and directories, but also a very important historic collection of scanned documents and manuscripts of famous cantors. Articles can be in German, Dutch or English. Yossele Rosenblatt's Recitatives, Sam Englander Piano Arrangements, Isaac Heymann's Psalms, and more.
http://www.chazzanut.com
Jewish National Library, Givat Ram, Jerusalem, Israel
The national sound digitization project has myriad musical examples online of various holidays and themes from the National Sound Archive housed in the National Library.
http://jnul.huji.ac.il/
Jewish Music Research Centre
The Jewish Music Research Centre of the Jewish National and University Library of Hebrew University on Givat Ram contain the National Sound Archives and several large special collections of Jewish music. Among these are the A.Z. Idelsohn Archives, the recordings collection of Robert Lachmann and recordings of music from Jews around the world.
http://shum.huji.ac.il/~jmrc/jmrc.htm