Sun has released the final version of its Open Document Format (ODF) plug-in for Microsoft Office, designed to allow the ODF standard to better compete with Microsoft's dominant Office formats.

The move comes just as Microsoft's Office Open XML (OOXML) file format is gaining the favour of organisations such as the Commonwealth of Massachusetts and the UK's National Archives.

Over the weekend Massachusetts reversed a pioneering policy of requiring users to use "open formats", originally defined to exclude OOXML, and agreed it will now consider OOXML an open standard.

Sun's plug-in is designed to allow organisations to use ODF while sticking with Microsoft Office. Microsoft is supporting ODF in Office via an open source plug-in of its own, but Microsoft's plug-in has important limitations, including the inability to save ODF files directly or use ODF as the default file format.

Sun had earlier released a preview version of the plug-in, but the final release adds support for Excel and PowerPoint as well as Word, and Microsoft Office 2000 and Office XP as well as Office 2003.

The final plug-in also improves conversion quality, Sun said.

Word 2007 is partially supported, but an error in the program's handling of filters makes it impossible to open ODF files, according to Malte Timmermann, technical architect for Sun's StarOffice/ organisation, and part of the original StarOffice team acquired by Sun in 1999.

"I hope Microsoft will fix this issue with the next service pack. If not, we will work around this bug by doing the same kind of integration like in PowerPoint and Excel," Timmermann wrote in ablog posting.

Sun is offering support contracts for the plug-in, and is working on localised versions, Timmermann said.

Unlike the Microsoft plug-in the Sun filter allows direct "control-S" saving into ODF formats and lets Word use ODF as a default format. Conversion uses tried and tested StarOffice code, which Timmermann said is better than Microsoft's XSLT-based implementation.

Also unlike Microsoft's plug-in, the Sun filter doesn't require any other software to be installed.

Supporters of ODF, an ISO standard based on the XML file formats used in StarOffice and, say the format could allow for open competition between different office application suites, since it's supported by different programs. OOXML, on the other hand, has practical restrictions that make it impossible to fully implement outside of Microsoft Office, according to critics.

Besides the competition issue, open formats are considered by many to be important for maintaining accessibility for older formats, though some archivists argue that open formats, like any other, will eventually become obsolete and need to be supported via emulation or migration.

The plug-in is available from Sun's download site.