Mattermost is a very modern approach to team chat, and offers both self-hosted and hosted options. It's written in Golang with a good chunk of JavaScript under the React framework. It features private and public chats, including one on one communication, good archival support, and a very similar interface to Slack, including most of the features you've come to expect there. In fact, if you're already using Slack, there's an easy import function which lets you move over your current channels and archives. Mattermost also integrates into your organization's existing LDAP or Active Directory authentication systems.