A friend recently introduced me to Wordstash. It's very easy to sign up for and just as easy to use.
I have introduced it to my Grades 4- 7 already and will be introducing it to our Teachers early next month. My desire is that the Teachers will bring their classes into the ICT Centre and allow them to create their lists each week, and the learners will be able to log on at home and either use the site to assist in learning or else create their own lists.
Another thing I enjoy about Wordstash is that it also automatically gives various definitions of the word, and allows you to either choose which definition you would like, or, alternatively, to add your own. In this way, the child is not just learning how to spell a word, they are also learning the definitions of each word.
One also has the choice of adding an image to the word. The site automatically sources images from either Wikipedia or Flickr for the learner to choose from.
When playing, they are presented with a variety of games and ways to test their Spelling.
Another way that this site could be used is just for learning definitions. I am thinking specifically of things like in English that one needs to know for language or poetry.