Number 1 on "My Top 10 Most Useful Free Programs" is Open Office available from http://www.openoffice.org
I know most people use some version of Microsoft Office to open and create word and xls docs but there is another free alternative. Open Office is "The free and open productivity suite". It has most of the same basic functionality as its MS counterpart. It even has the capability to open and save word and excel documents.
For those on a budget that would like to have standard office document capabilities seriously consider Open Office. I will admit that it may not have every advanced feature that the MS programs have but for the average home or even office user Open Office will be more than adequate.
There is also a portable version that will run from a USB stick. You can find more info on this version at PortableApps.com