I would echo what @40hz
...I use the Video Download Helper add-on for Firefox and I've been quite happy with it. Read more/download it from here.
I have DownloadHelper, and it integrates with GetRight (a downloading utility), which speeds up downloads of large files by segmenting files and downloading the segments in parallel. Works very well.
Having said that, if you have already watched a YouTube video, or even part of it
, then you probably don't need to download it as it will probably already reside complete in your browser cache.
There is a superb utility from NirSoft - VideoCacheView
- that scans your various browsers' caches and identifies media downloads - e.g., including YouTube .flv files. You can view the video in cache, check if it is all there, and if you want to keep it, then copy it to a folder, rename it and give it the correct extension, if necessary (e.g., ".flv").
What a lot of people (like me) did not realise is that if you "play" a YouTube online, and then press pause, it continues downloading into your browser's cache - unless you expressly tell it (rightclick) to STOP downloading. This is how come it gets into your cache. It is a potential and invisible bandwidth hog.
Sometimes the quality of image or sound of these cached files seems to be better
than the quality of the best
files you can download using a YouTube downloader - these latter can often sound very "echoey" too. So I occasionally use the cached files if I want to (say) strip the good quality sound out of a music video and save it as an .MP3.
I could be wrong, of course, but I suspect that this "echoey" quality of downloaded files could be deliberate spoiling of quality of downloadable product, to protect copyright or something. For all we know, Google has itself tied up in knots with RIAA and similar over DRM and restrictive practices, and we consumers will have heard little about it.
I can't offer much help on the subject of file conversion, as I rarely need to do that, and anyway, I have found that many of the conversion tools that I have played with have a "lossy" process, so you may lose sound/video integrity/quality as a result.