This is a great website to see what other teachers are using in their classes. Mainly activities! (You have to make an account but it is definitely worth it, and search "education" and you will find tons of great stuff!)
This social networking site could be used by educators much the same as Diigo. The one real difference is that Pinterest is a visual space (akin to a pin board) where you can get a sense of the content at a glance and dig deeper based on what you see.
This site is a good resource for teachers to find helpful information about any particular subject. This site is where my Crotched Mt. teacher found IXL to use for she students.