With an inexpensive USB OTG adapter, you can connect things like keyboards, USB hard drives, and even gaming controllers to your Android phone or tablet. Read this blog post by Sharon Vaknin on How To.
We’ve seen iPhone cases that comes with additional lenses that can be attached to it. This allows photographers to add lenses such as zoom, macro and wide-angle to their iPhone, allowing for a different level of photography.
Somewhere deep in the bowels of Logitech, a committee, attempting to think outside of the box, created a box. It's called the Cube (even though it's clearly not a cube) and it's a mouse. And a presenter.