Slack was late to the world of workplace messaging apps. When it arrived in in 2013, it didn’t appear to offer much that was new — chat rooms and direct messaging were already available in a host of apps. But its co-founder Stewart Butterfield cannily presented it as a powerful alternative to email and, over time, as a command hub for the workplace that would integrate all the other software a company uses into a single interface. Slack spread like wildfire, and today is worth at least $7 billion, The Wall Street Journal reported.

Now the company plans to make its stock available for purchase: the company filed its initial public offering today. It is expected to become one of the most valuable tech companies to go public since Snapchat.

It didn’t get this far without some carnage. Slack beat out tons of messaging apps on its way to the New York Stock Exchange, skillfully incorporating features that are fun (unlimited custom emoji reactions to messages) alongside those that are simply necessary (records retention features for large corporations).

One app it didn’t necessarily beat yet and that is arguably its biggest rival: Microsoft Teams. Microsoft has bundled Teams with Office 365 subscriptions, which becomes useful if you still use Word, Excel, and PowerPoint, instead of the Google alternatives. By making Teams free for Office 365 users, Microsoft has essentially made the service into an easy default for many, and that could pose a challenge to Slack. It’s also hidden just exactly how many users are on Teams, although we know 329,000 organizations are on the service, as of September last year.

Here are some of the rival apps that have come — and in some cases, gone — since Slack came around.

Internet Relay Chat

The grandfather of internet chat is even older than AOL Instant Messenger. It’s an open protocol, and so it’s still alive and kicking — but it just never achieved the scale and popularity that Slack has, perhaps because it was just a little rough around the edges.

Skype

In the early 2010s, Skype was a common choice for workplaces to send quick messages when needed. But as a sort of all-purpose messaging app, Skype never really made adjustments for its enterprise users and seriously lacked integrations with other services. For over a decade, it didn’t even have the ability to record calls. I remember Skype fondly for letting you place international calls for a couple of dollars, but it was never great for remote work. Its animated emojis were also super corny.

HipChat

HipChat has been literally eaten by Slack, which acquired it from Atlassian in July of last year. Slack looked very similar to HipChat when it launched — but HipChat was slow to add third-party integrations, and lacked the free version that made Slack a hit with tiny startups. Atlassian attempted to launch a true Slack competitor called Stride in 2017, but gave up on it after it got little traction.

Google Hangouts Chat

Google Hangouts Chat remains a cult favorite thanks to its unlimited messages, easy access to other Google services, and more visible discussion threads. But as a lightweight chat client, Google Hangouts Chat was never going to compete with an enterprise-grade solution like Slack. (That was supposed to be the job of Google Hangouts Meet, which is a different thing that definitely... exists.)

Facebook’s Workplace

Facebook launched a version of its social network for the office in 2016. But two years of data privacy scandals have taken their toll on Workplace, which has been slow to grow amid concerns about how Facebook uses our data.