OpenOffice

A complete suite of office applications that mirrors Microsoft's, including a word processor, a spreadsheet application, a presentation maker (think PowerPoint), a database creator (like Access), and even drawing and equation applications. OpenOffice is one of the crown jewels of free software, with pretty much all the bells and whistles that you would want in an office suite. You can even create PDF files from any of the applications, something Microsoft can't do. Additionally, you can open and save to Word, Excel, and PowerPoint formats. Absolutely amazing.

This is the one to watch, and could, ultimately, be the Microsoft killer. Most businesses are tied to Windows purely because they can't afford to give up MS Office. OpenOffice runs on Linux, OSX, as well as Windows, and it does everything that the vast majority of businesses need. It's rapidly being adopted worldwide by governments and school systems. For example, Brazil is in the process of fully adopting it in all government offices.

http://OpenOffice.org