http://cyber.pref.kumamoto.jp - this is the most comprehensive web-site on Kumamoto. It has a huge amount of information on the prefecture, including maps to many of the places listed in this guide.
http://groups.yahoo.com/group/kumamoto-i/messages - a searchable archive of messages posted by people living in Kumamoto - useful for looking up information about restaurants, events etc. You can also subscribe to the group to post your own questions/opinions.
http://www2.kumagaku.ac.jp/teacher/~masden/links.html - a really comprehensive link page - with dozens of Kumamoto web pages listed. There is information on accommodation, events, doctors, movie guides, Kumamoto-slang, maps and much more.
http://www.kumamoto-today.net - check out this website for up to date information on Kumamoto. Run by Neil Perejma - the site has contributions from Japanese and foreign residents living in the prefecture. Current events/festivals, weather forecasts, sightseeing, history and general information about the ken are contained.
http://www.outdoorsjapan.com - This is a fantastic website for information about outdoors activities in Japan. Everything from skiing to hiking, diving to paragliding is included.