When locating free resources to use in the classroom or for teachers to use, my initial search is Google. Once I have located a few that target my needs, I then explore the sites further. Sometimes the sites have referring links or even research articles that I can investigate further as well. If the site is a really good site that will be used in future, I save the site in my favorites.
There are book resources as well for websites. I located one of these in the UNI's children's professional development collection, Internet Kids & Family Yellow Pages 2001 (Polly, 2001). Since it is a 2001 edition, there are many sites that probably no longer exist. However, I did purchase 2009 Internet Directory (Averello, et al. 2009) as a resource for parents and students. I have not checked it out as much as I would like at some point.
Averello, V.; Belicove, M.; Connor, N.; Crew, A.; Gunter, S. K.; Wempen, F. (2009). 2009 internet
directory. Upper Saddle River, NJ: Que Publishing.
Polly, J. A. (2001). Internet kids & family yellow pages, 2001 edition. NY: McGraw Hill.