OpenOffice.org is a free, open source alternative to Microsoft Office and other major office suites. Visit http://openoffice.org/ to find out more about it and to download it.
Part of what makes OpenOffice.org so attractive (besides being free to download, use and distribute) is its cross-compatibility with documents, spreadsheets, and presentations created with Microsoft Office. OpenOffice.org Writer will open, edit, and save Microsoft Word .doc files. The same goes for OpenOffice.org Calc and Microsoft Excel .xls files, and for OpenOffice.org Impress and Microsoft PowerPoint .ppt files.
This page from Idealware offers a feature-by-feature strengths and weaknesses comparison between Microsoft Office 2003 and OpenOffice.org.
When OpenOffice.org is installed, it is set up by default to save Writer, Calc, and Impress files in their respective native (.odt for Writer, .ods for Calc, .odp for Impress) formats. These formats have been approved by the International Standards Organization as the international standard for office data.
This would be fine if everyone were using these standards. For now, the reality is that the majority of businesses and people that create and exchange documents, spreadsheets and presentations in the US do so using Microsoft’s software and standards.
This guide will show you how to configure the OpenOffice.org suite to automatically save Writer files as .doc, Calc files as .xls, and Impress files as .ppt. These files can then be easily opened and edited in Microsoft Office 97, 2000, and 2003.
This will enable OpenOffice.org users to exchange files with Microsoft Office users.
Begin by opening the OpenOffice.org Writer program. Once Writer is open, select Tools from the menu bar at the top, and then select Options from the bottom of the Tools menu as shown here.

Once the Options window opens, find the Load/Save entry in the left-hand column, and then click on the “plus-sign” next to it to expand it.
The first expanded entry under Load/Save is General. Click on the word General to highlight it and display the available options.
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Note the two “drop-down” menus under the Default file format heading at the bottom of the window. This is where the save settings will be changed as shown below.
To save OpenOffice.org Writer documents as Microsoft Word 97/2000/XP .doc files, click on the Always save as drop-down on the right and change the default selection from OpenDocument Text to Microsoft Word 97/2000/XP as shown above.
Next, click on the Document type drop-down on the left and select Presentation from the menu.
click image below to open enlarged in a new tab or window)

Next, click on the Always save as drop-down on the right and change the default selection from OpenDocument Presentation to Microsoft PowerPoint 97/2000/XP as shown above.
(click image below to open enlarged in a new tab or window)

Finally, click on the Document type drop-down on the left and select Spreadsheet from the menu. Then click on the Always save as drop-down on the right and change the default selection from OpenDocument Spreadsheet to Microsoft Excel 97/2000/XP as shown above.
In reference to the note provided which states “Not using OpenDocument as the default format may cause formatting or content to be lost.”, we have never found this to be an issue. To be safe, it is recommended to use the Save as… feature when editing files originally created in Microsoft Office. Save your edited version under a slightly different file name so you’ll have an unchanged original version saved as well.
Finish by clicking the OK button to close the Options window and save your changes.
If you’re using OpenOffice.org on Windows and have available hard drive space, you also may want to download and install the free Word, Excel and PowerPoint Viewers from Microsoft. These have two main uses for the OpenOffice.org user. You can then create a document, spreadsheet, or presentation in OpenOffice.org, save it, and then open it in the appropriate Microsoft Office Viewer to see how it looks formatting and content-wise. You can also use the appropriate Microsoft Office Viewer to open any file that you may receive that won’t open using OpenOffice.org. Though the Microsoft Office Viewers will not allow the editing of files, content can be copied out of the viewer and pasted into the appropriate OpenOffice.org application using the Copy Paste menus or keyboard commands.
The Ramblings of a Digital Educator blog has some great links to free online OpenOffice.org and Microsoft Office tutorials.
Edit (01/29/2007): Kaj Kandler’s blog also lists training resources for OpenOffice.org, as does his Plan-B for OpenOffice.org website. The screencast tutorials at Plan-B are free while the site is beta-testing.