Mattermost is an open source alternative to Slack, providing group chat, direct messaging, private chat-rooms and all the functions you need for a real-time team collaboratiom tool. It's so good we use it ourselves, with our hosting network in the UK and Switzerland.
Prices start at £2 per user per month for your own Mattermost team (or £1 per user per month for not-for-profits, charities, and organisations that meet our ethical charter).
As well as a slew of Slack-alike features it has several items that Slack is missing.
One account across multiple teams rather than a separate account for each teams means that users can communicate with each other across teams without switching content. (Also good for IT - you can manage all users in one place).
Threaded messaging means that you can comment or reply to a message which means that you can move in and out of a discussion without losing context.
Clickable hashtags (like Twitter) as a way of grouping and organising messages. This can be a powerful tool especially if you have a set of pre-defined hashtags that everyone uses.
Markdown support to format your messages quickly and easily in a few keystrokes - great for highlighting important parts of messages, structuring messages - you an even use code formatting with syntax highlighting.
Multi-language support is built in for Japanese, Spanish, French, and Portuguese - more languages are on the way.
And of course you can import your chat history, user accounts and even theme from Slack into Mattermost.
Open up real-time communication for your teams with Mattermost with no limits: unlimited file storage; no limit on number of searchable messages; and unlimited third party/custom integrations.