http://www.terryburns.net for many years. I have them separated by topic and a lot of my writing friends use it often. I originally put it there not only for that reason but so I could easily access it myself anytime and anywhere.
I'm afraid I don't have time to maintain it as I should but there are still a lot of good links there. These days the maintenance is being done by users who write me to say a link no longer works or to suggest a new link to be included. I get a lot of these from teachers who say their students are using the list. That makes me happy.
Many of the links are not an individual site but rather a list of links itself. I just had one suggested to me that is just that, a very good list of history site resources. The site is at http://www.mastersinhistory.net/the-ultimate-guide-to-free-history-resources-online.html and was suggested by Mary Hubbard who manages the content for the site. Their list of resources is much more exhaustive than the history links that I've placed there.
I did an estimate once that between the number of links I have collected there and the number of links that are on the various resource lists that are there that something over 10,000 writing related links can be accessed from that library. Small wonder that I don't have time to get in there and maintain the list and still have time to service my clients.