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Is this the app that is built into Android that lets you open Word documents?

It's not built into Android, but I think some Manufacturers (HTC?) ship with QuickOffice (or QuickOffice Viewer) pre-installed.

It's an app available in the Android Market that lets you open word documents.

Also Excel and Powerpoint... plus editing:)

It's also an extremely popular Symbian S60 app that many of us still use.