If you are watching a Flash stream (not youtube) then it is possible to save it to a FLV file.

At first you have to buffer the full stream that is the gray bar.
When it is full loaded, execute at first this to locate the flash buffer:

lsof | grep Flash

After this you get a list with some informations.
The first line defines the flash-browser-plugin and the number behind is the PID of your stream. After this you find your username and behind this you have two numbers and a letter. The two numbers defines the full buffer and our “File ID” then adobe splits the stream in more parts.

Get your Flash File with this command:

cat /proc/"PID"/fd/"File ID" > ~/Vidoes/"Name".flv

Now you have fun with your offline stream.

Update: if you execute the “lsof | grep Flash" and you don´t get an File ID. Then you have to do one more step.

Take your PID from the lsof and make a second lsof on the base of the PID.

lsof -p 11621 |grep deleted

After that cat or copy your Flash File and have fun:

cat /proc/"PID"/fd/"File ID" > ~/Vidoes/"Name".flv