This site has an activity which tests the students' knowledge of the different states in the United States. Students click and drag each state to its correct place on the map. Great for students who are learning about states and capitals.
This site had online talking story books that could be used on an interactive whiteboard. This could be done as a whole class or used in small groups of students.
This site has science activities that are broken down by grade level and topic. The activities are informative and would be fun for the kids to try after learning about a topic in science.
Here's a simple site, mainly for drawing and artistry, but, like Dabbleboard, you can embed the picture into your site. I like embeds because I believe they can enhance the online learning environment by putting all of your tools in one place.
This simple site is a nice IWB online activity where students can choose letters to spell different words based on objects that they see. I really wish I could embed it, perhaps into an interactive lesson plan, but the little kids K-2, will like it.