On this site teachers upload units, lessons, and other materials they have created. There are for sale, but there are many items that you can download for free.
Edmodo is billed as Facebook for educators. You can set up a classroom page and then post info about what is happening each week. Parents (or older students) can create an account and join your classroom page to see the updates.
Okay, so I kept putting off joining Pinterest because I heard how addicting it can be. I finally joined and yes, it is addicting, but it has so many wonderful ideas. I am just starting, but you can follow me at http://pinterest.com/math4parents/
NEW! www.airserverapp.com (Favorite)
The National Library of Virtual Manipulatives is organized by grade band and concept.
You can find videos on YouTube to teach you about a concept or to use as an introduction into something you are teaching your students. Most schools block YouTube, so you also need to use the SaveMedia site (see above).