Microsoft Office is the one set of software you can almost guarantee will be on any computer you touch. It’s been out for the Mac since 1985, 5 years before it was on PCs (as hard as that seems to believe today), and has dominated the word processing, spreadsheet, and presentation market long enough that’s it’s the de facto standard.
There is competition, most notably on Macs from Apple’s own iWork, but also from open-source office apps. OpenOffice.org, a Sun Microsystem project, was the most prominent free office competitor for years, but was then forked into LibreOffice after Oracle bought out Sun. LibreOffice 4.0 was recently released, with native versions for OS X as well as Linux and Windows, so it seemed time to take it for a spin.