.doc (Word 97/2000) is the de facto standard. Even without Office installed, Windows comes with WordPad which will handle it. Any Linux distro worth downloading has half a dozen applications that can read .doc. Fairly certain that OS X can even read it without Office.
There is OpenOffice, which is huge and bloated but free. It will, of course, save its files by default in its own format that can only be read by OpenOffice, so it's a worse option than .doc (which it can read and write if you specify that).
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There is OpenOffice, which is huge and bloated but free. It will, of course, save its files by default in its own format that can only be read by OpenOffice, so it's a worse option than .doc (which it can read and write if you specify that).