From mangahigh.com, I noted that it was launched within the last year and the site does not include research data about student achievement with the software. Red flags include presentation and content stressing the goal of improving test results. To quote the site: "Score better at exams: Mangahigh complies with UK National Curriculum standards and offers exam-oriented revision and training in order to achieve measurable performance improvement."
In contrast, the site recommended by Tom and Kim have been around for a while, include research data, and reference US mathematics standards.
Enjoy the free trial with your students if the topics you are teaching are on mangahigh in a way that supports students mathematics understanding. Your school should be wary of the costs involved, lack of standards alignment, and the undocumented promise of student learning.
There are many excellent mathematics learning tools online including NCTM's Calculation Nation gaming site (http://calculationnation.nctm.org/) and the lessons and activities found on the Illuminations site.
A difference between sites that are free and sites that cost is that the sites that cost also promise student success on tests. In reality, no one can promise that students will be successful and we all know that what students need is quality time engaging in mathematics.
Gaming sites do engage students—now they have to document that they improve students’ mathematics learning.
Have you seen Dreambox - http://www.dreambox.com/ ?