The program below makes use of some features of HTML5 (specifically the "Blob" object, the File API, and the "download" attribute of the "a" tag) to allow the user to load, edit, and save a text file on their local computer. As of this writing, it works in both Chrome and Firefox browsers, though sadly it requires a little bit of custom code for each.

To see the source code, visit http://thiscouldbebetter.wordpress.com/2012/12/18/loading-editing-and-saving-a-text-file-in-html5-using-javascrip/