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 chang...
Google account holders and non-holders can be set to access and/or collaborate on files/folders in real-time. Additionally, you can find files you've shared not only by filename but by person you've shared the files with.
Calligra consists of 9 tools. In addition to word-processing, spreadsheet and presentation software, it includes tools for project management, sketching/painting, database management, vector graphics, diagramming and brainstorming.
Zoho Docs has 2 paid plans - $5/user/month and $8/user/month that add additional functionality and increase the storage limit. Compared to standalone purchases, the subscription fees can quickly rack up, especially since they are on a per user basis.