SpellingCity is probably the most robust and popular site around for Vocabulary/Spelling. This is a tool that educators are using in classrooms of all grade levels to help improve student's skills. Best of all educators can find or create their own word lists and then integrate them into over 26 different games. This is a great and fun way to help students practice their Spelling & Vocab skills while learning at the same time. Also, SC has lots of resources for teachers to use as well as a free mobile app (i.e. Kindle, iOS, Amazon) that integrates w/ the site for mobile learning.
I highly recommend checking out SpellingCity by clicking here!!!
Below is a brief demo of their mobile app...
*SpellingCity is an advertiser of Technology Tidbits.