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.