In my video folder I have some music clips in .flv I would have liked to tag with information as artist, song title, album, cover, etc.
Is this operation possible ? Which programs are able to do that graphically (equivalent of EasyTAG)?
flvtool2 and yamdi are tools to manipulate flv metadata, they are both command line based tools though, and you do not seem to want that, also the metadata they can inject may not be what you want, see what yamdi can do here http://yamdi.sourceforge.net/, something about flvtool2 here http://www.inlet-media.de/flvtool2/ the kind of metadata they handle may not be suitable so test them.
While it is not so good to convert one lossy compressed video format to another, I have no choice but to convert some of my videos to the vastly superior .mp4 format, in doing this videos can be tagged with itunes compatible metadata quite easily with mp4tags from mpeg4ip-utils, the metadata so embedded in this format is widely supported, all major media players on Ubuntu are able to read them and these tags will be available in the right click properties Audiio/Video tab in nautilus too, mp4tags is command line based but Easytag-aac can tag .mp4 video files quite easily too, even the VLC media player has this feature. it is up to you to convert or not to convert.
If conversion is out of the question then one can use the Banshee media player as a manager for most video files, just import your files into the program then you can enter the necessary information about your videos in the banshee interface, much the same way you do your music, the only thing is that this information will be stored in the Banshee sqlite database, and not the file itself, not a big broblem in some cases.