git clone https://github.com/yourorg/ip-multiviewer cd ip-multiviewer mkdir build && cd build cmake .. && make -j$(nproc) ./multiviewer --source udp://239.0.0.1:5000 --grid 2x2

is secretly one of the most powerful free multiviewers available. It natively supports a wide array of IP feeds, including RTSP, SRT, and HLS. OBS Studio

StreamGrid is a powerful open-source desktop application designed specifically for managing multiple video streams in a customizable grid layout.

Uses significantly fewer resources than a full NVR like Shinobi or ZoneMinder.