I recently picked up an addition machine from lakeshore learning.
Initially all the kids wanted to do was push the buttons and make silly faces and patterns with them but once I sat down and played a game showing them how it works they were hooked.
They both love picking a "problem", figuring the solution and then pushing the button to see if they got the answer correct. (then we say "ding ding ding" like a bell ringing)
Re: Educational product rave:
I had them too. I didn't even know they still sold them.