Mattermost

What it is:

Mattermost is an open-source chat and collaboration platform.

What it’s used for:

• Team messaging in real time (channels, private messages, group chats).
• Sharing files (documents, screenshots, PDFs, etc.).
• Organising discussions using threads so conversations don’t get messy.

Why we like it:

• Full data control (great for privacy-sensitive work or politics, councils, NGOs).
• Works on web, desktop, and mobile.
• Highly extensible — bots, plugins, webhooks, automations.