Do you like to watch video clips of you favorite musician at YouTube.com? As you know, you can download the videos at youtube by using additional plugin for you browser. But the downloaded file will be a .flv file which is a flash video. If you only want to have the music file, you can convert the .flv file into .mp3 file by using a converter available online.
The online converter I use are FLV2MP3 and VideoConvertX, both giving the same tools to convert .flv to .mp3 file. VideoConvertX also provide conversion tool to other formats such as AVI, OGG, MP4, 3GP, etc. which give you more options other than mp3.