Based on Sun's StarOffice suite, OpenOffice.org offers a full-featured office productivity suite.

Yes, it is compatible with Microsoft's Office.

Believe it or not, there is viable competition for Microsoft's Office suite.  I have been using OpenOffice.org exclusively for over one year and, frankly, I can see no need to change.

Is OpenOffice.org right for your organization?  I do not know (though I suspect it is).  Drop me a line and we can discuss it.