Trig Tour is one of the excellent HTML5 Math PhET Simulations from The University of Colorado Boulder. There are numerous PhET simulations covering Physics, Biology, Chemistry, Earth Science and Mathematics. Note the collection of HTML5 versions which will work across all platforms and devices.
For an excellent introduction to Functions the PhET Simulation, Function Builder provides a very visually appealing resource. Using the pattern option even very young students could explore important concepts. This is very intuitive to use, the slides in the following slideshow demonstrate examples of its use. Note that teachers can register with PhET and gain access to supporting documentation.
You can download all the HTML5 sims so they are available offline in an app. Available for iOS and Android for only $0.99 / 89p which supports HTML5 sims.
PhET Balancing Act working nicely on my phone!
Staying with Mechanics, one of my favourites now available in HTML5 collection: Projectile Motion.