Office Web App is part of your Hotmail or Office 365 subscription.
This is the main URL for the online version. But a customer can deploy it internally for their SharePoint use.
https://view.officeapps.live.com/op/view.aspx?src=
You can view the file by adding a public URL of the file and the Office Web App URL
Example: https://view.officeapps.live.com/op/view.aspx?src=http://webserver.domain.com/directory/filename.pptx
Let me know if you have any questions.