Not sure about HD videos from hd-porntube.com, but free SD videos from this site use HTTP protocol. You can download or capture them with freeware Fast Video Download, Freecorder 4, Jaksta Free Media Recorder, commercial Replay Media Catcher, WM Recorder as well as many other programs.

See also:
FAQ: How to record/download/capture/save Adobe Flash .FLV video HTTP stream from embedded player