We check this website, now excelviewer.herokuapp.com is up and dont blocked for our system and other users of the Internet.
So, answer to question: Is excelviewer Down Right Now? - NO! excelviewer is up!
If you have problem with access to site excelviewer, and domain excelviewer.herokuapp.com is not avalible.
Clear browser cache and/or try other web browser
(Chrome, Firefox or Safari).