Attention math teachers: mathebook.net looks like it could be a good review tool, practice tool, or summer activity. Mathebook.net has virtual math practice options for all levels (though there are more for Kindergarten, Elementary, and Middle School than High School). Mathebook.net also offers editable pdf worksheets for each concept which could be emailed or posted to a class wiki/blog, completed by the student, and submitted again via email, a drop.io site, a class drop box, etc. Especially nice, no special program is necessary to edit the pdf worksheets.
Mathebook.net could be a great tool for summer practice, class review, or a green way for practicing many math topics.