I got an email with an attachment in webarchive extension. When I tried to open it on a Windows computer, I first used Word, but Word could not open it. Renaming the extension to the popular formats like .doc or .pdf did not help either.
Wikipedia has an article about webarchive. The format is supported by Safari browser. Every mac has Safari browser built in. If you use a Windows machine, you have to download and install the Safari browser from Apple. Note that only version 4 beta (or above) supports webarchive format.
Try it and let us know if it works for you.