Open Office is an open source "office software suite" for spreadsheets, word processing and more. It is basically like Microsoft Office but free and without the hassle. It stores everything in an easily readable international standard format. It is available in many languages and allows for easy migration of already created documents.
Open Office is very big on the fact that it is a community of people there to help each other out. No expensive phone calls, just one big happy friendly community. As it is open source you can manipulate and add to it any way you please.
Find out more on their "Why" page: Why Open Office