Fully open source (including relay server although it's a minimal implementation lacking security) remote desktop software alternative to TeamViewer, works very similarly, easy for both parties to use. Cross-platform (Windows, Linux, MacOS, Android). No need to create an account, it works by sharing an automatically generated ID and password. Very fast and good quality streaming remote desktop agent (tested with 1080p YouTube videos), 2nd only to Remotely in open-source remote desktop agents. It also streams audio. Supports transparent bidirectional clipboard.
If you want an exactly similar experience to TeamViewer but using a FOSS software, RustDesk is the one. I highly recommend, and it has become my backup software for remote desktop control or for people who have a hard time dealing with agents that are different from TeamViewer's ID/password system.
Tip: reduce bitrate (custom graphical options in RustDesk options) and reduce the target machine's resolution and you'll have streaming performances similar if not better to the NX protocol (NoMachine), but RustDesk does not have a native option to do that.