The Redwall Encyclopedia
Snowfur no longer updates this site, sadly, but it has a list of characters from A to Z--literally--from each book up to Loamhedge. Plus all the pictures of the characters she could find in and out of the books (meaning foreign editions, etc.).
Redwall Headquarters
This site hasn't been updated since before Triss came out. However it has mountains of the poems, maps, and other things from the first 14 books, plus fan desktops.
Fort Marcanea
I'm not sure whether Fort Marcanea is still updated or not, however it has music and other downloads from the TV series and books and is a pretty-well set up site.
JackProcher.com
The official composer(s) for the Redwall TV series. There's some great music here!