The United States Department of Education website is a great online resource to explore. There are, for example, lesson plans and ideas, categorized by content areas. To get to this part of the website, go to features and then teaching resources.
The Alliance for Excellent Education is another great online resources. The highlight of this sites is probably the publications page. (Publications available include The Reading Next Report and The Writing Next Report.)
Another great resource for online lessons and ideas, check out Teachers' Domain, produced by public television.
The Teaching Tolerance website is rich with resources for teaching about acceptance and diversity.