Many online video sites such as Youtube, Google Video, Dailymotion, Metacafe, and others only provide limited or inconvenient access to the videos; either they require you to install the proprietary Flash player (and I surely won't do that), and/or you can only view them online (but not download them).
There are some solutions, each with advantages and disadvantages:
- youtube-dl, a command line script to download Youtube videos.
- metacafe-dl, same as above but for Metacafe videos.
- clive, a command line tool (with optional newt UI) for Youtube, Google Video (seems defunct at the moment) and Dailymotion.
- VideoDownloader, a Firefox plugin which is supposed to work with more than 60 video sites (Youtube and Google Video are among them). The only disadvantage compared to the other solutions: you need to start Firefox + X11 (no pure command line usage).
- UnPlug, a Firefox plugin similar to VideoDownloader, but with the advantage that it doesn't use the VideoDownloader web service (but rather figures out the video URLs itself).
- Gnash, a free software Flash video player is another option, but AFAICS it's not yet ready for daily usage (but it's getting there).
- swfdec, another Free Software Flash player, is actually working quite nice with Youtube already.
After the download, you can either view the videos using (e.g.) mplayer, or recode them into a more sane format. For all of the above programs there are Debian packages available, except for VideoDownloader/UnPlug (but you can easily install those from within Firefox).
Update 2007-07-26: Added UnPlug and swfdec (thanks Joe Buck and Josh Triplet for the comments).