Iain Lamb's Typograph is cool tool that allows you to explore the relationship between scale and rhythm by modifying the properties of typographic elements in a page of sample text.
It sets out to explore how two, intertwined concepts, often playful but sometimes cheeky, can be encouraged to dance in web pages. Drag the colored boxes along the scale to throw these words anew. For the most part, this text is just a libretto for the performance you are about to play upon it."