Here are some of the web-based tools that I find most useful in my teaching practice:
Padlet allows for the quick creation of custom web "walls" that allow students to post and share.
Socrative is a great web-based assessment tool.
SMART Exchange is a place for teachers to share and search for SMART Notebook-based lesson plans and ideas.
GeoGebraTube is the place to explore all things GeoGebra.