Element
United Kingdom
End-to-end encrypted Matrix-based team messenger and collaboration suite.
European Alternative to:

A chat app built by the team behind Matrix, the open protocol it runs on. The company started in 2017 (Matthew Hodgson and Amandine Le Pape, originally as New Vector) and is based in London. The Matrix idea is that messaging should work like email: different servers and apps talk to each other instead of locking you into one silo.
What you get
- End-to-end encryption for messages, plus voice and video calls
- Federation, so your server talks to anyone else's; run your own (Synapse) or use Element's hosted version
- Bridges to Slack, Discord, Telegram, WhatsApp and IRC, so you can keep a foot in those without leaving Element
- Spaces for organising rooms, plus threads, replies, reactions, and the rest of the usual team-chat toolkit
- Clients for web, desktop, iOS and Android
Where it's used
Because it can be fully self-hosted and audited, Element has been picked up by governments and large institutions across Europe. France's Tchap and the German Bundeswehr's BwMessenger are both built on Matrix. The software is open source; Element's hosted service and enterprise server suite are the paid offering.
Honestly
Matrix is powerful but can feel rough next to Slack. Encryption key management and the odd bridge quirk are real. The upside is that you actually own the conversation: your data, your server, your rules.