HandBrake is a popular free and open source video transcoder available for Linux, Windows and Mac OS X. The tool works great however, the app widgets are black under Unity, this issue being caused by Ubuntu's overlay scrollbars:

HandBrake, a free, open source video transcoder for Linux, Windows and Mac OS X, has been updated to version 0.10.0, getting support for new encoders, like H.265 and VP8, along other interesting changes.

HandBrake is a cross-platform video transcoder that supports most common multimedia formats, DVDs or Bluray-like sources that aren't copy-protected. The application can even create chapters or add subtitles.

After more than a year, a new HandBrake version has been released: 0.9.6, and it includes many improvements, such as:

Arista Transcoder is a really cool video converter for GNOME. It comes with a huge list of presets: from iPod, iPhone, PSP, Playstation 3 to Android, Nokia, DVD and lots more. It supports MP4, WebM, Matroska, AVI, Ogg, FLV (codecs - video: H.264, VP8, MPEG4 / DivX, MPEG2, Theora, Flash Video / audio: AAC, Vorbis, MP3, FLAC, Speex, WAV) formats and it can rip straight from DVD or v4l (Video 4 Linux) devices too.

Dirpy Studio is a new web service that you can use to extract the audio in high quality (mp3) from any YouTube video. You can also use it to download the video in mp4 format (for mobile devices).

All you have to do is insert the video link - if you know it, or you can perform a search directly from Dirpy to find the video that interests you. In the Transcoder section you have a number of options like extracting the entire audio track or only a part of it, the quality in kbps, even add id3 tags such as artist name, album, etc. And finally, there is a video options section where you can download the video in flv (medium or low quality) or mp4 (high quality) formats.

Dirpy also comes with a bookmarklet which you can click whenever viewing a YouTube video - clicking the bookmarklet will instantly open up Dirpy with the option to download the sound in an mp3 or as flv or mp4 (video). To use the bookmarklet simply drag it to your bookmarks toolbar:

Dirpy: YouTube to mp3

and whenever you want to transcode an YouTube video - click it.