SDR Touch is an Android app which allows the rtl-sdr dongle to be used on an Android device. A USB OTG (on-the-go) cable is required to connect the android device to the tv dongle. The blurb on the app authors website reads
(SDR Touch) Turns your mobile phone or tablet into an affordable and portable software defined radio scanner. Allows you to listen to live on air FM radio stations, weather reports, police, fire department and emergency stations, taxi traffic, airplane communications, audio of analogue TV broadcasts, HAM radio amateurs, digital broadcasts and many more! Depending on the hardware used, its radio frequency coverage could span between 50 MHz and 2.2 GHz. It currently demodulates WFM, AM, NFM, USB, LSB, DSB, CWU and CLW signals.
If you do not have an rtl-sdr dongle yet, you can find a good place to grab a compatible one for about $20USD from the Buy RTL-SDR page.
Currently, it seems that USB, LSB, CWU and CLW are experimental and may not fully work. In a recent update squelch, gain and offset control have been added to the app. SDR Touch is still early in its development and as such many features may still be missing. A list of upcoming features and supported tuners are shown on this developers forum thread on xda.
Most newer devices with Android 4.0 or higher should work with SDR Touch. Devices with older versions of Android may work if rooted. Some Android devices also may not provide enough power through the USB port to power the tv dongle. A USB OTG cable with an extra power port may be required to power the dongle. An extra power port may be a good idea regardless, as an app like SDR Touch is prone to draining the battery fast.
SDR Touch has been having some licensing issues which has resulted in its removal from the Google Play store. It can still be downloaded for free from the authors website. Upgrading to the Pro version for £5.99 removes a trial timer on the graphical spectrum viewer feature.
A lot of people seem keen to use SDR Touch as an Android in car radio on their tablet. The SDR Touch author has promised to integrate more features such as a Frequency Manager which will help with this use.
Here is a test with a rtl-sdr R820T dongle on a HTC One X Android Jellybean phone
Test with a rtl-sdr R820T donge on a Motorola Xoom
Test with an unknown DVB-T rtl-sdr dongle on a Nexus 7
And here is a SDR Touch video review