Is there anything that can share screen between more than two people (like e.g. TeamViewer), but is open-source?
At the very minimum, one user should be able to present their desktop, exact copy of what they are seeing, to one or more other users. Giving the remote user control permission would be nice. Chat/voice/file transfers and similar are not needed; I can always use XMPP/Jingle or something like that for those.
TightVNC is a cross-platform (Linux and Windows) open source implementation of the VNC protocol. There are many other VNC clients and servers, most should work with each other without problem. Most Linux distributions have some VNC server/client built in, so does OS X (called Screen sharing).
Jitsi is a set of open-source projects that allows you to easily build and deploy secure videoconferencing solutions. At the heart of Jitsi are Jitsi Videobridge and Jitsi Meet, which let you have conferences on the internet, while other projects in the community enable other features such as audio, dial-in, recording, and simulcasting.
BigBlueButton is a web conferencing system designed for online learning
BigBlueButton enables you to share your audio, slides, chat, video, and desktop with students. Built-in polling makes it easy to engage students and recording your lectures means that you can make them available for later review.
When using the whiteboard tool in BigBlueButton, annotations are automatically displayed back to the students in real-time. Presenters also have the ability to zoom, highlight, draw and write on presentations making your points clearer to remote students.
Mconf is an open source web conference system built on top of (and around of) BigBlueButton. Mconf is composed of several components, among them are Mconf-Live, a customized version of BigBlueButton that includes several new features, and Mconf-Web, a web portal where people can collaborate asynchronously, schedule and participate in web conferences. Keep reading this page to learn more about Mconf!