OpenOffice is a popular free (open source) productivity suite that is roughly equivalent to iWork or Microsoft Office.
iWork can open some OpenOffice documents. For example, click on the Open Office .doc file and it drag it onto the Pages icon in the Dock. The file will open as a Pages document.
However, .odt or .fodt text documents don't open in iWork/Pages.
If you encounter this problem, don't panic. TextEdit, the simple word processor that comes pre-installed on Macs, can open .odt documents. Click on the file, drag it onto the TextEdit icon in the Dock and it will open.
You can then work on the document in TextEdit or copy-paste it into Pages if needed.
Read about OpenDocument format on Wikipedia here and about OpenOffice here.