A javascript function and a jQuery extension for maximizing font sizes to fill out the element's width.
textStretch.js is minimal, fast and robust. It doesn't require or add any wrappers or child-elements, and it adapts to when you resize the window or flip your device.
textStretch is built with older browsers in mind. It's tested and works with Chrome, Safari, Firefox and Internet Explorer (I haven't tested any version older than 6 though).