Early Music FAQ
This is the place to start! Among (many) other things, this site has general information on medieval music ranging from the basic to the complex, an index of early music CDs, discussion of instruments, and sources for music (both online and off).
Ceolas Celtic Music Archive
Another comprehensive guide, with a more narrow focus. Includes information on instruments, dance, artists, tunes, and other resources.
Recorder Home Page
Everything, and I do mean everything, you ever wanted to know about the recorder.
Medieval Recorder
Part of the previous site, but most immediately SCA-relevant. Has several illustrations of period recorders and period art showing recorders.
Playford's Dancing Master
Complete page facsimiles of the widely-used (although slightly post-period) dance manual.
Acadia Early Music Archive
Downloadable scores in PostScript format (with links to free software for viewing and printing the scores). There is a lot here, although most of it seems to be "formal" rather than "folk" music.
And if this isn't enough to get you going...
Medieval Music
A collection of annotated links to other information.