The SVG (Scalable Vector Graphic) file-format is an XML-based vector in that can be used on the web right now. Not only are they crisp at any size they are scaled to, but they are supported by IE9+, Firefox, Chrome, and Safari. For IE8 and below, you can provide a fallback PNG. Simple icons and logos are good candidates for the format, as they are often the first noticeable elements of a design that will look blurry on a retina display...