1. Try Science
This site offers teachers and students a variety of experiences: Adventures (offline thematic experiences), Experiments (offline activities with online components), Field Trips (Interactive science centre features and locators), Curious (short polls and activities based on topics) and Live Cams (various live webcams for science centres). There are information pages for Parents, Teachers and Scientists and Engineers.
The experiments are organised according to learning areas (Mathematics, Space Science etc). The Try Online option allows you to view the experiment in an animations, while the Try Offline option details how to carry out the experiment. These experiments are contributed by various science centres and are very detailed. There is also a section where kids can comment about the experiments.
You can follow the New York Hall of Science on Twitter and Facebook.
Science kids also has 'sister' for Mathematics and English.
If you don't already follow NASA or NASA APOD on Twitter or Google+ I suggest you start the magnificent pictures and videos to generate discussion and wonder.
You can followQuestacon on Twitter.