I don't know much about that. But gstreamer is the classic tool for video loading / decoding / displaying. Look at the list of plugins , they have several "videosink" (which is the lingo for "displaying the video"), I believe it can be easily be used with OpenGL, SDL, X, Xv, OSX, DirectX, and other less well-known rendering systems.
Then, there is also the well-known VideoLAN (VLC). You can use libvlc to embed video into either an SDL surface or a Qt widget. With all the video formats that VLC supports.
Then, phonon which can be used in Qt4.5 and above.
There are plenty of options to choose from.
OpenCV is another which can easily grab raw video and display it on a separate window.
The list could go on... This depends also one whether you want to decode video files or if you want to display/record a raw video input (IP camera, firewire, etc.). VLC is probably the most comprehensive solution. But again, I don't know much about video.