Platforms: Windows, Mac, Linux
This office suite is distinguished by its traditional and familiar interface and also by the fact that it includes working equivalents of Microsoft Word, Excel, Publisher, Powerpoint and Access. This is probably the most useful free and open source package you will find.
LibreOffice has excellent compatibility with many formats – from proprietary Microsoft formats to lesser known ones – and is constantly evolving (one of my favorite small additions last year was the inclusion of dynamic word count at the bottom of the screen).
The next step we will be waiting for is some kind of cloud storage!