This retracktable sandstacker works in the current version of minecraft, in multiplayer survival.

Most of the credits go to: Minecraft SethBling as this design was based on his video:
https://www.youtube.com/watch?v=62o3FzBpneY

However I had to make some repeater adjustments to have it work correctly, the left lever will make it stack left, the right lever will make it stack right.
Please note I've also altered it so that it can be extended into rows, which will enable you to have bridges and stairs to appear out of seemingly flat or hidden locations.

You need:

2 levers
32 repeaters
32 redstone
4 sticky pistons
2 pistons
18 blocks (any solid block that can have redstone on it and doesn't fall).
a bunch of sand

First level Layout with repeater settings is:
        
     ***BB*1BB1*BB***
     *   **    **   *
     2   *3    3*   2
     1   *B    B*   1
     1   31    13   1
     *111**    **111*
     *111 BS  SB 111*
          12  21
          *L  L*

Second level layout is:

        *BBBPPBBB*
         B      B

Third level layout is:

         *1S##S1*

*   = Redstone
1-4 = repeater (1 is default)
B   = Solid Block
S   = Sticky Piston
P   = Piston
L   = Lever
#   = Sand

You can extend it by copying the main row.

All the best!

FliP