Confluence is a popular collaboration platform released by Atlassian in 2004. Because it is wiki-based, Confluence is now considered outdated as the trend of moving away from wikis to social collaboration has been going strong for a while. Bitrix24 has roughly twice the features available in Confluence, comes with a modern social interface and is free for 12 users. Here's a feature comparison chart to help you decide if Bitrix24 is a free Confluence alternative you've been looking for.
* Please note that the information may have changed since the publication. For current prices and features please visit Bitrix24 pricing page.