S4A:

Scratch for Arduino (S4A) is a modified version of Scratch, ready to interact with Arduino boards.

Supported boards
S4A works with Arduino Diecimila, Duemilanove and Uno. Other boards haven't been tested, but they may also work.

Connectivity
Components have to be connected in a particular way. S4A allows for 6 analog inputs (analog pins), 2 digital inputs (digital pins 2 and 3), 3 analog outputs (digital pins 5, 6 and 9), 3 digital outputs (pins 10, 11 and 13) and 4 special outputs to connect Parallax continuous rotation servomotors (digital pins 4, 7, 8 and 12).

You can manage a board wirelessly by attaching an RF module to it, such as Xbee.

S4A allows you to control as many boards as USB ports you have.

Compatibility
S4A is backwards compatible with Scratch, so you can open Scratch projects in it. However, you won't be able to share your projects on the Scratch community website because doing so goes against the Scratch terms of use.

Take in account that this compatibility doesn't work both ways, so you won't be able to open an S4A project from within the original Scratch.

Using a PicoBoard along with an Arduino board is also supported... cont'd