Directions This is a nice and easy application for keeping track of your favorite locations and places. You can then share these with your friends by sending them with normal text messages. You can also send your own location making it easy to find each other anywhere. It's also possible to take photos that get geotagged and placed correctly on the map.
Fluids This is a fluids simulation application for the iPhone (and of course the iPod Touch). The iPhone is a brilliant device for this kind of application because you get instant feedback and nice interaction using the multitouch screen and inbuilt accelerometer. Drag your finger across the screen to inject new liquid and/or stir around what is already there. Tilt the device to make the liquid react to gravity. You can load your own images (or take new ones using the camera) to use as the base texture of the fluid. You can also save out an image of the fluid whenever you fancy!