What are some good, free, software tools for internal corporate communication?
Here are some free, popular software tools for internal corporate communication:

- Slack: A collaboration and communication platform that offers real-time messaging, file sharing, and integrations with other tools and services.
- Microsoft Teams: A collaboration and communication platform that is part of the Microsoft Office 365 suite, offering messaging, video and audio calls, file sharing, and integration with other Microsoft tools.
- Google Workspace (formerly G Suite): A cloud-based suite of productivity tools that includes Gmail, Google Drive, and Google Meet, as well as other tools for collaboration and communication.
- Mattermost: an open source, self-hosted alternative to Slack that provides team communication, file sharing, and search capabilities
- Rocket.Chat: a self-hosted, open source communication platform that supports voice, video, and text messaging
- Zulip: an open source, real-time group chat platform that offers threaded conversations and integrates with a variety of tools and services
- Matrix: an open standard for decentralized communication that provides a secure, decentralized alternative to proprietary chat platforms
- Riot.im: a free, open source collaboration platform that provides secure, encrypted messaging, voice and video calls, and file sharing.
- Zoom: A cloud-based video conferencing platform that offers audio and video calls, screen sharing, and recording capabilities.

These tools offer a range of features, from basic messaging and file sharing to more advanced collaboration and integration capabilities, so you can choose the one that best fits your company's needs.
which ones are completely free?
All of the options I've provided are completely free of use, with the caveat of storage limits, upload limits, or limits on the services.
If you are referring to free as in free and open source, consider using these services:

- Mattermost
- Rocket.Chat
- Zulip
- Matrix
- Riot.im