Open-source alternatives to SharePoint for small/medium businesses?

I'm looking for an open-source alternative to SharePoint for a small workgroup. The main thing we need is reasonably robust document management/distribution, but other features (workgroup calendaring, etc.) would be useful. We're using a wiki now to distribute docs and we need better metadata, version control, etc. The solution needs to run on Linux (ideally). The obvious candidates seem to be Alfresco Community Edition or Liferay, but I have zero practical experience using or administering either. Thanks in advance, hive mind, for any tips or advice you can offer.