I have long been a big fan of Google applications. My school has been using Google Apps for some time, particularly Gmail, Docs and Sites. However, more recently I have been promoting the use of Apple products and apps, with the focus being on the iPod Touch, iPads, and the use of apps on these two devices.
Over the last couple of days I have been reading a number of articles on the new Nexus 7; it really does look like a potentially game breaking device to use in schools - but with one significant drawback; the limited range of applications available for the tablet.
Although the iPad 2 (which my school uses) is significantly more expensive than the Nexus 7, this is more than compensated by the huge range of apps (either free or very cheap) that can be utilized on the iPad and the (far less expensive) iPod Touch.
For me, the iPad and iPod Touch are the most effective mobile devices to be used in schools.