Hello, overhere you can find my self building bridge.
It's made of pistons, redstone and water and lava.
Eventually a cobblestone bridge is built.
The only thing you have to do is press the button, the machine will start. You never have to stop the machine, the pistons will stop automaticly. when you break a blok or a creeper or a block of TNT explodes, the bridge automaticly starts building again.
Hope you enjoy it :)

How to create a cobblestone or smooth stone generator: Youtube Video

!!UPDATE!! [6 January 2012]
Design looks the same, but it's faster.

!!UPDATE!! [8 January 2012]
Sorry guys, unfortunately I made a big mistake while uploading the previous update.. everyone who downloaded it since then.. there is a chance it didn't work.. Just fixed it, it'll work now..

Additional Notes

Please leave a comment and tell me what I should improve =)