At the risk of sounding like a shill, I'd like to shill Zulip. We started using it instead of Slack, and it has been amazing. It's fast, everything is designed to help you get to conversations quickly, keyboard accessibility is second to none and the streams is a much better default than rooms.
I can't recommend it enough, it's well worth the money even though they have a nice free tier and are OSS so you can self-host.
That's one thing I'm currently missing in Ripcord (the Slack client). Or at least I haven't figured out how to do it. I use Slack from three devices, and sometimes I read a message I want to reply to later from a different device - marking messages as unread was a handy way for ensuring I don't lose them.
I'll just go ahead and admit to shilling. Zulip is amazing and I tried to get my company to switch to it. The only thing that stopped us from switching is well, the fact that people have to switch. They have to learn another tool, and Zulip sadly is not too friendly when you first start using it, though it's amazing for power users.
I literally believe it's the future of enterprise chat. There's no better tool for getting actual work done.
edit: I guess I should clarify about the shilling part. I contributed a single feature to zulip, once. So I guess I'm not really a shill, but I love the product.
Keyboard accessibility is second to Weechat. Weechat responds instantly to all keypresses and never leaves janky UI on the screen.
When reading Zulip threads I frequently mute those I'm not interested in. To do so, you have to hit 'i' to open a menu, then 'M' (there is no shortcut on 'm'), then a popup message slowly animates in saying the thread is muted. If you hit 'n' to go to the next thread, the popup message doesn't clear, so it's covering the top message of the new thread, and the popup menu is still open covering the top 1-3 messages, still listing the last thread's title. So leaving a single thread I know I'm emphatically not interested in takes five seconds. There's a nasty positive feedback loop: catching up on threads is slow and frustrating, so I leave it longer, so there's more threads that take even more time and frustration, repeat.
I reported this buggy workflow ~2 years ago and it's why I've repeatedly given up on a very interesting community's Zulip chat. There's plenty more flaky stuff, like switching messages with j/k sometimes not scrolling the screen, or long messages where you have to take your hand off the home row to scroll up and down because if you hit space the floating nav covers lines you haven't seen, long messages being "closed" so you have to navigate to each individually to read, no keys are customizable, death by a thousand cuts. I have my fingers crossed for a bitlbee integration.
Zulip looks great, but it also looks like the sort of thing I'd have trouble convincing everyone to switch over to because even though it might be more useful in the long term, in the short term its a bit more complicated (even though we're on the free slack, so we're looking to move elsewhere)
That's exactly how I'd explain it. It took me about 3 hours to be extremely comfortable using it and feeling productive. But first impressions by all of my coworkers were "It's so confusing", "There's no way we can use this, it's terrible", etc. But it's amazing. Just absolutely fantastic.
I've been hosting a personal server, a company server, and servers for every one of clients since we started as a web consulting company.
It's far superior to Slack for remote work in our experience because it's designed to be primarily used asynchronously, as opposed to something like Slack/IRC where you have a wall of unread messages all mixed together every time you sign in.
The biggest difference is that every conversation must have a topic title and be put into a stream, giving all the benefits of forums or email threads with subjects. Compare that to Slack/IRC where the the default mode is putting your messages in big firehose channels that don't encourage breaking individual conversions out into separate threads.
Zulip's hosted plans for corporate users have a similar model to Slack's -- there's a free plan with limited access to history, and a paid plan with full access to everything.
Since you mentioned nonprofits, we provide free hosting for open source projects and free or highly discounted hosting (e.g. 15% price) for many other nonprofits.
User interface, information density. Zulip is very verbose and cluttered and after a while it becomes difficult to find information from past conversations, things scroll out of screen very quickly, etc.. And there doesn't seem to be much of a way to customize it, short of going for the source code. For IRC there's a ton of customizability around in the form of various clients, scripts, plugins, etc.
I can't recommend it enough, it's well worth the money even though they have a nice free tier and are OSS so you can self-host.