Calligra Suite is an open source Office Suite which comes packed with features. It currently works best on Linux, though: the OS X and Windows builds are variously described by the developers as "preliminary", "highly experimental" and "unstable", so you can download and try the suite, but should expect problems.
If you're happy to try the suite anyway, though, this is what you'll find.
Words is the Calligra word processor. It can open Microsoft Word DOC and DOCX files, ODT/ ODM documents and assorted other formats, and can save as ODT, ODM, EPUB, HTML and PDF, amongst others. And there's a similar level of Office compatibility elsewhere, so Sheets (the spreadsheet application) can read XLS/ XLSX files, and Stage (the presentation tool) can handle PowerPoint documents.
Other productivity tools on offer here include Flow for flow charting and diagrams, Plan to handle your project management needs, while Kexi provides a visual way to create and manage databases.
And if you're just feeling creative then you also get Karbon, a capable and extensible vector drawing tool, and Krita for painting and sketching.
How does this all feel in real life? It really does depend on your platform. The Linux builds work well, but when we tried the suite on Windows it was a very different story, with all kinds of odd problems appearing (much as the developers warned, to be fair). Words wouldn't launch at first, for some reason. Application windows couldn't be resized. Poorly chosen colour schemes made it extremely difficult to read some menus. The Help system just displayed errors about missing files, and so it went on.
Calligra is certainly ambitious, then, and if you're running Linux then it already provides plenty of Office-compatible power. If you're running Windows, though, we'd stick with OpenOffice, instead.