There is VideoDownloadHelper for Firefox, ClipGrab for YouTube and Vimeo and if you still use an older Safari version, Safari's Activity Window, and there is some more:
KeepVid - a website, that offers you to download streaming videos as .flv, .mp4, WebM, .3gp and even .mp3.
ClickToFlash - a Safari plugin and/or extension, to block Flash, but also download Flash videos using the .mp4 format, though it does not work on all sites.
Safari's Activity window - use Safari and its Activity window to download Flash embedded video, does not work on all sites.
Evom - Download Flash videos from websites to iPods, iPhones, iPads, and many other devices.
ClipGrab - free downloader and converter for YouTube, Vimeo, Dailymotion and many other online video sites.