Try Algebra Trees for example. Form inputs, operations and output by dragging them onto the main workspce, connect them up and optionally connect to a graph. Click inside any of the elements to change the content.
Once you have tried a few of these applets you will find them intuitive to use.
Algebra Arrows for example could be used to compare different orders of operations. Build a tree, make the input x and note the output generated:
There are several applets which are excellent for showing plans and elevations, see for example Building Houses with side views where the user must construct a house given top, fron and right views. Teachers familiar with the ‘Improving learning in Mathematics’ materials will recognise some of these applets.