AbiWord is worth mentioning in the context of these other programs -- it's a free and open source word processing application that continues to be developed and maintained by a community of users. Its feature base is not as broad as OpenOffice.org -- it's only a word processor, not a whole suite of applications. But that may also be a major point of appeal, since it only takes up about 15MB on installation and runs with amazing swiftness.
AbiWord is essentially a clone of the "classic" Microsoft Word's core features.
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As the name implies, AbiWord is essentially a clone of the "classic" Microsoft Word's core features. Many of the common things you'd use Word for -- letters, reports, mail merges, revision tracking -- are all available here. It's not suitable for really professional use, but students and individuals who aren't doing enterprise-level work will find it just about perfect. It's also available cross-platform, so if you get started using it on Windows, you'll be able to take it with you if you switch to Linux.
Classic Word documents (Word 97-2003) open as-is, and a plugin's readily available for OpenOffice and ODF files, but there's as yet no support for Office 2007 OOXML. Not everything opens perfectly, though: when I tried opening a Word document with multiple languages (English and Japanese, in this case), the non-English text didn't render correctly. To that end, AbiWord is probably best for creating entirely new documents instead of dealing with existing legacy ones.
The roster of clever little features that go with AbiWord is endearing. The program's export-to-HTML function lets you package any images in the document within the HTML file itself, using base64 encoding. Third-party plugins add their own gamut of functionality,too -- mostly import/export of various file formats, but the formats supported include things like LaTeX, WordPerfect and Clarisworks. Last but not least, you can run AbiWord in a PortableApps package.
It's hard to go wrong with OpenOffice.org as a default choice. Aside from enjoying the support of both Sun and IBM (albeit in different ways), it's expanded its cross-compatibility with Microsoft Office, making it that much easier for people to migrate and continue existing work.
Symphony is quite polished if not as fully loaded as the original OpenOffice.org suite; it's going to be worth keeping an eye on this to see where it goes when it enters the 2.0 and 3.0 phases. StarOffice's main draw is the support and deployment features that go with it, but those are for-pay only -- albeit at a very reasonable per-user cost.
KOffice has more media/graphic designer tools, but that much less of a prospective audience due to being Linux-only. And finally, AbiWord is small and light, convenient if you just want a word processor and nothing else.