You can't learn a dance you haven't seen from dance notes, but they can be helpful for remembering dances you've been taught and forgotten.  If you know of other such sites, please let me know about them in the comments section below.

The Folk Dance Federation of California has a good number of dances notes, many from original presentations by the dance collectors: http://www.folkdance.com/html/LDDescriptions.htm

Stockton Folk Dance Camp publishes the notes from dances presented at their workshops: http://www.folkdancecamp.org/syllabi/

Tom Deering's web site has some of our dances on it:  http://www.seanet.com/~deerings/dnotes.htm

Andrew Carnie has an interesting dance notation method: http://folkdancemusings.blogspot.com/p/alphabetical-list-of-dance-notes-on.html

Evansville dancers: http://www.evansvillefolkdancers.com/dance-notes.php