BigHugeLabs Mosaic Maker could easily be used in the classroom. I envision using it for a classification assignment or maybe even to create hands-on activities. Students could use it for projects on genetic disorders, bacteria & viruses, etc.
Wordle has lots of personal uses, but could also be easily adapted for the classroom. Kids could use this for vocabulary terms or to review for a final. It would be fun to put all of the vocab terms learned together at the end of the year to show kids just how much they have learned - they'd probably be amazed!