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LibreOffice is the free power-packed Open Source personal productivity suite for Windows, Macintosh and Linux, that gives you six feature-rich applications for all your document production and data processing needs: Writer, Calc, Impress, Draw, Math and Base. Support and documentation is free from a large, dedicated community of users, contributors and developers.
SpecsUpdate
Pros
Pro The most popular FOSS Microsoft Office alternative
Out of all free and open source alternatives for Microsoft Office, LibreOffice is the most popular and with the most community support behind it.
Pro Good Zotero integration
LibreOffice works very well with Zotero. The integration of the Zotero plugin in LibreOffice is a breeze, citing while writing and generating the bibliography is just a click. Also good since you need to coop in an (MS) world: Libreoffice and Zotero cooperate very well to make sure your refs survive a "Save as" from one file format to another (e.g. docx to odt or the other way around). Basic (open and free!) tools is your in the market for this...
Pro Is either preinstalled on distros, or available through official repositories
Distros like Fedora, Salix, etc.. come with LibreOffice preinstalled. If you're running another distro, there's almost certainly an official repository available to install LibreOffice in the easiest way possible.
Pro Ribbon design
Since 5.3, LibreOffice offers a Ribbon design view (similar to Microsoft Office) (check here).
Cons
Con Does not support MS Office formatting
Most documents created in MS Office look different when opened in LibreOffice.
Con Track Changes needs to be improved
Collaborating on a single document is difficult. Track Changes makes the document hard to read as both edited and deleted text is displayed, it's not possible to print comments, tracked differences between document versions are sub-optimal and there's no system for accepting/rejecting changes.