LibreOffice has long since moved on from being an offshoot of the OpenOffice project. It's now the go-to alternative to Microsoft Office, enjoying regular development and a constant tweaking to keep it up to date, stable and secure.
While LibreOffice lacks the slick presentation of rival Windows freebie Kingsoft Office Suite Free and its Office-inspired ribbon interface, it has the major advantage of offering a full set of office applications, including database, drawing package and mathematical tool on top of the requisite word processor, spreadsheet and presentation tool found in most free office suites.
Version 5 makes much of its user-interface improvements, but while it’s definitely more navigable than before thanks to better placement of tools and visual previews of styles direct from the main toolbar, it still looks a little dated, despite the refreshed icon sets. Nevertheless, this is still a powerful suite capable of meeting most people's needs, making this zero-cost alternative to Office a valuable tool to have in your armoury.
Ever-improving support for third-party file types (including both new and classic Office formats) means you're not cutting yourself off from industry standards while cross-platform support means you can run LibreOffice on Windows, Mac or Linux without having to retrain yourself to a new way of working each time.
Note this is the portable version of LibreOffice.