Transtar
Addictive! Figure out where the line of reflection is and click it to get the shape to reflect. Beware of Red Giants. The best part of this game is it trains you as you go. Very cool!

Not as cool as Transtar, but still good practice
Inserting reflection lines.

Adding Positive and Negative Numbers
Discover the hidden picture.

Adding Positive and Negative Numbers
Line Jumpers, a FunBrain Game.

National Library of Virtual Manipulatives

Mirror Manipulative
Line of reflection