Drag and drop your truetype (ttf), opentype (otf), scalable vector graphics (svg) or Web Open Font Format (WOFF) fonts in the left hand side module and it will be added to the list. The last font dropped will change the font-family of this text and the above title.
At the moment this functionality of dragging and dropping files from the desktop to the browser is only available in Firefox 3.6. You can expect to see this added to other browsers in the near future.