Are you looking for free SharePoint Foundation 2016, 2013 or 2010? Alas, after Microsoft announced that they weren't going to make it available in 2016, your list of free alternatives isn't going to be long. Here's why Bitrix24 is probably the best SharePoint Foundation alternative right now. First, Bitrix24 is absolutely free in cloud for unlimited users and you can purchase on-premise editions as well at a fraction of regular SharePoint costs. Second, you get the same set of tools - intranet, document management, collaboration, workflows, tasks and projects, email, instant messenger and so on. Third, Bitrix24 comes with native iOS and Android apps.
* Please note that the information may have changed since the publication. For current prices and features please visit Bitrix24 pricing page.