There is also a mailing list (email listserv@listserv.aol.com with the message "subscribe NINPO-L"), an IRC channel (#bujinkan), a Usenet group, and many bulletin boards (e.g. Kutaki no Mura), although in all of these uninformed gossip often detracts from serious discussion.
In-depth information is easier to find offline, through English and other non-Japanese books as well as Japanese books. If you'd like to see Dr Hatsumi in action in your own home, you can order his Bujinkan videos or the 'live' Taikai videos.
Those interested most in the "Ninja" side of things may find Iga City interesting, enlightening or amusing... Finally, if you want to research other martial arts you might as well start at dmoz.