Cool sites to use, but be careful of Terms of Service. Some need logins, some are not for under 13. Some may no longer be free, or free for only a lite version.
Check for educational versions.

Google Apps for Education

WikiSpaces
Link to sign up for an Ad Free wikispace

Jeopardy Labs

Teacher Tube

Tux Paint
Open Source painting software for kids.

Sumo Paint

Top10freesites ( an older site )
Created by Steve Dembo

Animoto

Animoto for Education

Voicethread

Kerpoof online picture creator

Wallwisher no changing to Padlet, digital post it communication

Wordle word graphics

Go Animate animation creation

Glogster Edu interactive posters

Extranormal text to video

Dipity online timelines

Polleverywhere

Prezi

Web 2.0 sites alligned to ISTE standards

ZimmerTwins - video creation