(Don't pay attention to the background, just the moss stone, the DB and the torch matters here)
![Image](http://uppix.net/6/2/c/c8cdc0774e94c5e5cd5b2014e0b7a.png)
All you need is a BD, with one glass block and one solid block in its inventory. Place a redstone torch right under the block that will be placed by the BD.
Connect the BD to a torch, and a button to that torch.
Press the button, the BD will switch to the next block in its inventory.
The glass block won't pass the current, the solid block will :
![Image](http://uppix.net/a/3/3/a501a552747938d882d3ef27adbdc.png)
It can also be used as a counter (up to 9) thanks to the sequential order of the BD !
Enjoy :)