1. Video Conferencing - https://jitsi.org/- been great alternative to zoom for socials (not restricted to 45mins) and the video quality for me has been above all the others.
2. Virtual Whiteboard - https://excalidraw.com/ - responsive and fast method to share ideas and gain mutual understanding of concepts.
3. Collaborative writing - https://etherpad.org/ - takes a while to get used to working together on a single document but when you do its a very productivity way to produce short documents and emails - avoids back-and-forth editing between team members as this is done in real-time.
There are obviously enterprise offerings for the above but really impressed with these open source offerings and want to know if I am missing some. If you want to go beyond top 3 that's fine also :) keen to hear as I am sure others are.