Scratch

There are different options for connecting a Scratch project to external devices.  The Pico Board is preconfigured with sensors and can be used out of the box with Scratch. 
http://www.playfulinvention.com/picoboard.html

Another option for creating your own devices is the GoGo Board, which you would build around your own design.
http://www.gogoboard.org/cocoon/gogosite/home.xsp?lang=en

The GoGo Board works with the current version of Scratch (1.3).  Here are the steps for setting it up to work with Scratch:

a.)  Connect the GoGo board to your computer
b.)  Start Scratch
c.)  Tell Scratch that you are using a GoGo board
      1- Select the "sensing" category
      2- Right-click on the "slider  sensor value" block and choose "show ScratchBoard watcher" from the  menu. A sensor watcher widget will appear in the stage area but it  will say "off" and all the values will be zero.
      3- Right-click on the sensor watcher while holding the shift key.  This gives you a special "expert" menu. release the shift key, and  select "use GoGo board" from the menu.
d.) Power-cycle your GoGo board to make sure it is reset
f.)  Right-click (without the shift key) on the sensor watcher widget and choose "select serial/USB port" from the menu, then choose  the COM or serial port to which the GoGo board is connected.  The displays should change to show values--probably mostly 100  or 99 unless you have sensors connected to the GoGo board

There are a few steps, but they should be quick to go through!