SharePoint is a Jack of all trades, master of none so based on you requirements you have to choose the right tool most of which are OSS or at the very least non depended on Windows
SP is a mesh up of several software suits slowly coming to getter, it is getting better every release. SP could also be the right choice depending in your needs. Mainly work group portals but those tend to fail (as in not being used) since most of the time there is no proper business case nor business processes in place to support the grand idea