Project Draw is a browser-based diagraming utility that lets you draw to your hearts content, without the need for plugins of any kind. You can import a DWF file or image. You can export the file as a DWF, PDF or graphic.
Check it out: http://draw.labs.autodesk.com/ADDraw/draw.html
Instructional video: http://www.youtube.com/watch?v=ZN60Ob5x7f0&feature=user