Slack-alike w/ no social features, corporate focus

Chanty - Soulless and corporate. Business pricing.

Pumble - See above

Twist - See above

Wire - Open source (AGPLv3), but still designed to fill the role of Slack, not Discord.

SMS-type, no space organization + lack of features

Briar - Technologically imcomprehensible to typical users. Also not really made for high-bandwidth use in peacetime.

Meshtastic - Designed around special hardware most people will not understand how to set up and probably don't want to buy.

SimpleX - Open-source project, favorite of a number of computer freaks. Basically the same thing as signal but without phone numbers.

Viber - Closed source. Japanese comapny, designed for domestic use but has an English transaltion. Basically Japanese WeChat.

Zangi - Telegram / Whatsapp if they had fewer features.

Open source but centralized server w/ no federation

Nermity - Open-source project, started from 0. Meant to mimic but not be a 1:1 clone of Discord. Extremely strict moderation.

Sharkord - Very recent project (Oct 2025), currently in open alpha.

Spacebar (previously Fosscord) - Open-source reverse-engineering project based on Discord's API.

Inapplicable

X / Twitter - has a very basic messaging function but nothing else. Also owned by the world's most pathetic crybaby loser among other issues.

Twitter-alikes or other microblogging / ActivityPub platforms - Bsky, Mastodon and its forks (Pleroma, etc.), PixelFed, Lemmy, PeerTube, so on and so forth. These sorts of platforms may or may not have direct messages, and for the ones that do, they are usually an afterthought included as a necessity, not as the primary purpose of the service. The only reason Steam is in the main list and not down here is because most people have an account already and it offers a number of chat features that other chat-second platforms don't (like voice calls).

Legacy technology

Email - Would only serve as a point of comparison. Modern instant messaging is a fundamentally different paradigm than SMTP could ever support, even with the ability to self-host email servers.

IRC - While all modern chat clients are based on IRC in some form, the original spec is from 1988, and the most recent extant IRC client was created in 2009 (HexChat). Image / file sharing requires extensions or third-party services. Basically nobody coming from Discord would accept such a drastic reduction in features.

SMS / texting - The standards for SMS have come to include things like group chats over the years, and it's possible to send images and files (and take phone calls), I don't think anyone wants to use this. Phones are infamous for constant spam calls. Cellular plans are expensive. People really don't like sharing their phone numbers. It's just generally a bad idea.

Web forums / BBS - Similar situaton to IRC. Although forums are still in use (even if Reddit has killed most of them), they aren't really suited to the kind of communication people use chat clients for.

XMPP - Also similar to IRC, created in 1999. Essentially IRC++. Extremely feature-rich with file-sharing, group chats, encryption, P2P voice / video calling, etc. It's an open standard with loads of community implementations for both clients and servers. It's an excellent choice for people who are willing to host their own server, but that's the primary issue with it. Despite all the cool features it has that bring it in-line with even Matrix, it's inaccessible to most people, as you need to find a server to host you (like jabber.org). Normally that wouldn't be terrible, just a step backwards (similar to what Matrix does), but due to the way extensions work, most people will need to host their own server in order to have the experience they're looking for (like message retention not being active on most servers, or only being turned on by server admins for select rooms, see XEP-0313), which involves all the time and effort investment you'd expect.

Other reason

DCTS - Open-source project from August 2023. Designed to mimic Discord while being self-hostable. Each hosted server acts as one "server" would in Discord, one channel list, one user list, etc. This however means users cannot make their own servers / spaces. Users from any server can federate to any other server, however.

Osmium - Closed-source Discord-alike. Currently in open alpha (beta?). For-profit company. Few people are talking about this one, probably because of the parallels to Root.

Quiet - Open-source, P2P client with channel organization, functions over Tor. 4-ish year old project (December 2021) but lacks most features due to being P2P. Seems to have fallen out of favor due to signal existing, even with the lack of channel organization.