Slack, the team communication app, went down earlier this week, sending people to Twitter to commiserate. So what if the startup is valued at $2.8 billion? It’s still a web service, and web services have outages sometimes.

Of course, there is Internet relay chat (IRC), but that’s a protocol. Ultimately Slack can be thought of as a hosted and souped-up IRC client, and there are plenty of other ones to choose from.

Here are five full-featured Slack alternatives — tools that go beyond IRC, in other words — that are open-source software, which means you can download it and run it on whatever server you want. That implies that you’re in charge of security, for better or worse, instead of, say, Slack.

Friends

Friends.

Above: Friends.

Image Credit: Friends

This tool emerged earlier this year, Friends stands out for its ability to let people communicate with othersĀ on the same local network, even when there’s no Internet connection.

Kaiwa

Kaiwa.

Above: Kaiwa.

Image Credit: Kaiwa

Based on the XMPP messaging protocol, Kaiwa was released earlier this year by French software development shop Digicoop.

Mattermost

Mattermost.

Above: Mattermost.

Image Credit: Mattermost

Available under a GNU AGPL license, Mattermost the platform has been selected by startup GitLab to ship alongside its eponymous open-source code-repository software. Mattermost the company is preparing to launch an enterprise-grade version of the open-source software.

Rocket.Chat

Rocket.Chat.

Above: Rocket.Chat.

Image Credit: Rocket.Chat

Established earlier in 2015, Rocket.Chat has a wide range of capabilities, like file sharing, video conferencing, and service-desk messaging.

Zulip

Zuplip on Mac and desktop.

Above: Zuplip on Mac and desktop.

Image Credit: Zulip

Dropbox acquired the team behind Zulip last year and released the Zulip software under an Apache license this past September.

There are other options out there, but these have gotten traction in the open-source world, so they’ll probably continue to be around for a while.

Slack itself has not open-sourced its own client. If that changes, I will of course add it to this list.