jQuery TextFill is a jQuery plugin resizes text to make it fit into a container. The font size gets as big as possible.
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Fitter Happier Text replaces each node with an SVG text node and sets the viewBox attribute based on its width and height.
TextTailor.js is a jQuery plugin for responsive text to fill the height of the parent element or ellipse it when it doesn't fit.
Squishy is a jQuery plugin that automatically resizes text to exactly fit the container with no extra work on your part. This plugin only works for a single line; if you are looking for multiline text fitting you can see SlabText.