Slack is a group chat and business messaging application that has been growing in popularity rapidly, especially in tech circles. It comes with a clean interface and is very intuitive. Slack disadvantages include the inability to simultaneously work in different accounts, lack of videoconferencing and no self-hosted edition. Bitrix24 is free for unlimited users and doesn't have any limitations on history search or the number of integrations available to free users. Here's a side by side feature comparison chart to help you decide if Bitrix24 is a free Slack 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.