The jQuery HTML5 Audio Player with Flash Fallback.

jQuery CSS Fallback Plugin

This plugin provides a fallback, for example, if you use a CDN for a CSS file.

jquery replacesvg

this is a simple svg replacement plugin.

jQuery SVGMagic

A simple jQuery plugin that creates a fallback for .SVG images (backgroundimage supported to!). In case your browser doesn’t support .SVG images it will create a .PNG file and replace the .SVG with it. This means that you don’t have to create two versions of your images to support IE8 and lower.