The Arduino Science Journal (formerly Science Journal, an initiative by Google) is free and open-source, and allows you to gather data about the world around you by harnessing the sensors in your smartphone as well as sensors connected to Arduino, or other third party hardware. The Science Journal transforms smartphones and tablets into science notebooks that encourage students to explore their world.
With built-in device sensors as well as external hardware, you can measure light, sound, movement, and much more. You can also compare results, and even set triggers.
Together with the external hardware, (not included with the app), students are enabled to conduct more complex experiments, and advance in their scientific studies. As long as the external sensors are compatible with a Bluetooth-connecting device such as a microcontroller, there is no end to what experiments the students can make. Some popular sensors that the app can work with are: light, conductivity, temperature, force, gas, heart rate, respiration, radiation, pressure, magnetism, and many more.