If you find yourself giving presentations often (or even doing photo slideshows), I strongly suggest you get one of these. I used an earlier version of this for the foss.in keynote and totally fell in love with it. Later the foss.in team gifted me this new model and I have been addicted to it since then.
It has a nice toggle button to move through your slides, dedicated buttons to change the volume, a Thinkpad style trackpoint which can be used to control the mouse and of course a very useful laser pointer. Best of all, the USB fits right behind the device, so you do not have to carry them separately. This device totally changed my lazy movie watching experience too.