Lego WeDo - see detailed page here
Makey-Makey
Students love the Makey-Makey. Minimal instruction is needed. The BIG lesson from this is learning to complete a circuit and discovering what conducts electricity and what does not. Simple but fun!
I like to just start by showing students the fun introductory video on the the site http://www.makeymakey.com and then asking them to follow instructions on http://makeymakey.com/howto.php and once they have reached Step 5 - they can begin to experiment.
A big box of stuff (some that are not conductors) and access to *any* program. We do like to use our own Scratch programs, it just makes it a lot more interesting and also encourages students to improve the code.
Pico Boards
Not sure if these will be as useful as they used to be earlier, now that you may also invest in the Makey-Makey. They do connect to more items and I believe you will need a software download for them to work on Scratch 1.4. Need an extension download to work on Scratch 2.0
See http://www.picocricket.com/picoboard.html
LEAP controller
I have access to a LEAP controller but have not found time to try this out. Works with the offline version of Scratch 2.0, see https://airspace.leapmotion.com/apps/scratch-2-0-plug-in-for-leap-motion/osx
Arduino
There is a Scratch For Arduino S4Arduino, see http://s4a.cat but have not yet tried it out. I believe it requires a download first to the Arduino.
This is a very useful project since it makes the Arduino much easier to use for younger students.
See more at
http://wiki.scratch.mit.edu/wiki/How_to_Connect_to_the_Physical_World