iCoolsoft FLV to MP3 Converter for Mac is a commercial converter. It allows you to convert FLV and SWF files into audio files encoded using a variety of audio codecs. The supported output formats are AAC, AC3, MKA, MP3, WAV, AIFF, AMR, AU, FLAC, M4A, OGG, and WMA. Those are the most popular formats used these days.
The main use that I would give this application is to rip the audio track off video files downloaded from YouTube or similar video sharing websites. I know of people who download music videos and videos of live performances and then convert them to MP3 to listen to those on their MP3 players.
Graphically, the application looks like any video converter. There is a space for your conversion queue on the left, a video player on the right, and the conversion settings at the bottom of the screen. There is a trimmer as well, which you can use to cut your video into a smaller video.
In my testing, the application worked like a charm. I converted an FLV file that I had downloaded from YouTube into an MP3 file which played great on iTunes.