Open Office. While it's not *AS* feature rich as MS office, I think it's good enough for 99% of users. Only thing I don't like about OO is the very limited selection of colors, like for borders, fonts etc in Impress (did not check if it's the same case for Writer). I still use Office 2003 I find it's pretty good, the versions above that are way too bloated though.
What sucks is I am seeing more and more of pptx and xlsx and those weird formats. Nothing (even the ms viewers) seem to open those properly.