http://www.commonsensemedia.org/app-reviews

A nice site with reviews of over 2000 Apps.

You can search for apps by subject, topic, age range and skills.

Detailed reviews of each app with concentration on the quality of the app and the learning involved.

Click on the title of an app to see lots of detail about the app.

Only downside is the prices of the apps are in dollars which can mean that some of the apps may not be available in the UK.