Ok, so I read in Notch's blog about him implementing piston mod in the future. So I thought, oh well, I might as well try it as a mod and once he releases it as official, i will just update the game and there will be no change.
So what is so awesome about pistons? Well they move blocks. So far the only thing I used them for was to create a "door" to my standard 3X2 ways. I always that that making corridors 2X2 feels claustrophobic, but then you can make double doors.
Also double doors have one big flaw about them. On one side it is opened, on the other side it is closed because the dor on one side is inverted in comparison to the other side. Thus if you make a simple circuit to open and close them at ones, one of them will always be opened. Of course it can be solved with an inverter, but as you know inverters have a delay, so one door will always lag a bit when opened and closed.
However I made a "door" of blast resistant pistons. Here it is:
when open
So far I have been messing around with pistons only one day, but from my experimentation I found out that they can control lave and water flow, they can launch players into the air, they can push any blocks except obsidian, they can push carts and many other things that I have not figured out yet.