Buta-Connection: tombeau des lucioles [French]
http://www.buta-connection.net/films/hotaru.php
This is my favourite Grave of the Fireflies website. All the sites at Buta-Connection are well done and filled with interesting information. Here you can find an analysis of the movie, the production information on the film, a history of the film background, and a great deal of information about the original novel. The link I posted above takes you directly to their Grave of the Fireflies site, but as a result it eliminates the frames that suppose to be part of the page; you can enter through the main page at http://www.buta-connection.net.

Nausicaa.net
http://www.nausicaa.net/miyazaki/grave/
Nausicaa.net offers scripts, an FAQ, and information about all the film-related products available. You should also visit the site if you want to learn more about the director Isao Takahata.

Onlineghibli.com
http://www.onlineghibli.com/grave_of_fireflies/
You can find a really great synopsis of the film here. In addition, you can download the original movie trailer for the film made by Studio Ghibli. Also has a gallery of screenshots from the film.

Untitled Hotaru
http://www.angelfire.com/pq/seita2flies/
Provides information on the movie, the book, and the director. Has a very large gallery of images from the movie.

Tux_Ramen's Unofficial Grave of the Fireflies Page
http://setsuko.8m.com/
Information on the movie, a gallery, FAQ, and script.