I played around making a stupid thing a bit like sqlfiddle but it would animate the executuon plan to visually show how rows were joined etc. (Say showing 10-20 rows of data at each stage), could also show stats for full dataset being processed.

I thought it would be handy for those learning sql, but i dont think its a pro tool.

In anycase i am mainly posting this comment as it seems a simple thing to do but ive never seen another tool that does it.. maybe if i had taken the project a few steps further i would have found a reason its a terrible idea.