Video encoders that display frame with most movement
This feature is called "motion detection" in the video surveillance jargon and can be found on most IP cameras and NVRs. There is a large variety of solutions out there depending on the level of accuracy that you need, the ability to divide the monitored perimeter into zones, whether the analytics run off a live or recorded feed, the OS that you use, and more.

If you look for something free and simple that just feeds you with XMLs then you might check Shotdetect. If you look for more of a "solution" then check motion, note that it is unix-specific.

It is also possible to use ffmpeg to extract thumbnails from your feed and run SNR comparisons on the fly, if you extract only I-Frames then the overhead is minimal. You are bound though to a detection rate that depends on your keyframe rate.

Try to provide some more detail regarding your operating system and overall use case for a more specific answer.