The balance strategy
This Java Applet on the Freudenthal Institute for Science and Mathematics Education (FIsme) website allows exploration of solving linear equations with the unknown on both sides using the balance strategy.
Learners can devise their own questions as well as solve those given by the software. There is little difference mathematically between doing this with pencil and paper and using the Applet, but the Applet appeals to some learners as it removes the need for writing and also has a games-like feel to it. Learners can also deal with errors immediately.
The Applet does not provide any justification for the balance method, and in particular it assumes that learners will understand how to deal with negative amounts within the balance metaphor.