OxygenOffice Professional is a free open source office suite, based on OpenOffice.org.
The core of the program is essentially unchanged from OpenOffice, which means you get a Microsoft Office-compatible word processor, spreadsheet and presentation tool, with database support and a drawing program thrown in for good measure. Every module reads and writes the appropriate Microsoft Office documents, so for example Writer, the word processor, can handle files in DOC, DOCX and ODT formats, as well as RTF, TXT and more.
What OxygenOffice Professional adds are extra touches, like a gallery of clipart and photos that you can drag and drop into any document. There are extra templates, more fonts, new sample documents, and more.
The suite also includes many low-level tweaks. There's better EMF rendering, for instance, and enhanced SVG support. You get updated import files for Office Open XML, Works, WordPerfect, WordPerfect Graphic and other document types, and performance is enhanced across the suite.
There's a new tool for accessing and making better use of Wikipedia, very handy for research projects (as long as you're suitably sceptical of the material, of course, and take time to review it). And if you keep browsing then you'll find interesting new features everywhere, so for instance the calculator now includes both a new solver, and VBA macro support.