I noticed recently that Google mail allows you to view attachements as HTML. This is a great feature. The last thing I want to do when reading an email in my browser is to launch Word. Google mail allows you to view the following file formats as html: .pdf, .doc, .xls, .ppt, .rtf, .sxw, .sxc, .sxi, .sdw, .sdc, .sdd, and .wml.
Missing from their list is the OpenDocument format which was recently adopted by the State of Massachusetts as the required format.