OpenOffice.org

OpenOffice.org is a free, multi-platform office suite. It includes key desktop applications, such as a word processor, spreadsheet, presentation manager, drawing program, and database module. OpenOffice.org is an Open Source project and available on all major platforms, including Linux, Mac OS X and Windows. At present, you can choose from more than 100 different language options.
OpenOffice.org avoids the risk of vendor lock-in because it uses a standardised, XML-based file format that makes interoperability easy and your documents future-proof.
OpenOffice.org is compatible with Microsoft Office. The latest version imports .docx files and continues to be able to read and write all the other Office formats.

Get OpenOffice.org

“OpenOffice.org is released under the LGPL licence. This means you may use it for any purpose—domestic, commercial, educational, public administration. You may install it on as many computers as you like. You may make copies and give them away to family, friends, students, employees—anyone you like.”
download.openoffice.org

Submitting Bug Reports

“Simple end users are really critical in the process of making the code better. It’s important for end users to understand how important they are in the process.” OpenOffice.org developer Florian Reuter discusses some of the issues and explains how you can take part.

Office 2007 to gain native ODF support early next year

“Users will be able to set ODF as their default file formats under Office 2007, and Microsoft plans to continue support of the OOXML-ODF translator for those using older versions of Office.” Jacqui Cheng details Microsoft’s recent announcement to make Office 2007 compatible with OpenOffice.org.
Personally, I remain sceptical about Microsoft delivering on these promises. But genuine interoperability has to start from somewhere…
arstechnica.com

Users of Microsoft Office may also want to consider the Sun ODF Plugin for Microsoft Office as an alternative solution.