A new simulation in the excellent HTML5 collection – from PhET Interactive Simulations, we have Area Model Algebra, this is very simple to use. Rectangles of various sizes can be built showing the relationship between multiplication and area.
Also new in the collection, I like both Area Model Decimals and the Equality Explorer series.
The ability to display or hide the variable and to take snapshots means working can be clearly shown.
I have included a PhET Simulations page in the Lesson Planning series which notes several favourites I have used many times in the classroom.