Google Documents can be be embedded directly using the Google Sites Insert (Embed Feature). With Google Sites Insert (Embed Feature) you can embed Google Docs documents like Spreadsheets, Charts, Diagrams, Drawings, Presentations, Forms and many more . 

OpenOffice or OO formats are support in the Google Docs Cloudware. You can also Sync OpenOffice with Google docs with a Google Chrome Extension.

Other documents such as pdf files and Third party formats can be placed in a IFrame tag. Flash SWF animation can be placed in a Object tag (Allowing both object and embed for higher browser support)

Some Embed examples for Google Sites and Third Party Embed Examples.