I use Libre Office. Open Office was purchased by Sun (Oracle) a few years ago and the developers who originally wrote Open Office moved to Libre Office.
I figure that at some point Larry Ellison is going to start charging for Open Office and just make a mess out of it.
That being said, They are both good alternatives to MS Office.
You should be aware that both Open and Libre office use a file format called ODF which MS Office sort of recognizes. You can go into the preferences and set Open and Libre office to use the XML (Office 2007-2013 format) for Writer, Spreadsheet and presenter (Word, Excel and Powerpoint).
One advantage to Open Office/Libre office:
They have versions that run on Mac, Windows and Linux. Since I have all three environments, the interface is the same for all three. Much the same as Firefox and Chrome for browsers it makes the transition from Operation system to operating system easier.
As to the 20.00 that someone was referencing for MS office, If you work for a company (Large one) that has an enterprise agreement with Microsoft, Microsoft the agreement allows employees of the company to install MS Office on their home machine for Home Use.
Technically it's free, but some states have issues with it being a benefit to the employees and tax the corporations for the "benefit" they are offering their employees. The Great Tax and Fee State of California is one of them. To extend the Home Use program to employees, you link to the Microsoft site and pay them 10.00 (Downloaded version) or 20.00 (You get the DVD) for MS office and somehow that money makes it back to the states coffers.