Onavo

Anyone who’s ever accidentally downloaded a large email while on holiday will attest to the ridiculousness of data roaming charges, and though there’s no indication from the networks that they’re working on putting things right, there are measures you can take to avoid an end-of-month sting. Once installed, this app drastically reduces the amount of data required to perform everyday tasks, such as retrieving email and posting to Facebook. We’re not entirely sure how it manages such a feat – we just know that it works and we’re not about to complain.
Available on iPhone (free) and Android (free)