Curlew is an easy to use, Free and Open-Source Multimedia converter for Linux.
MystiQ
Easy to use and elegant Multimedia converter.
pdf2png
Convert PDF books to multiple images in various image formats.
DeaDBeeF
DeaDBeeF lets you play variety of audio formats, convert between them, customize the UI almost any way you want, and use many additional plugins which can extend it even more.
HandBrake
HandBrake is an open-source video transcoder.
Converseen
With Converseen you can convert, resize, rotate and flip an infinite number of images with a mouse click.
Ciano
The ciano is a multimedia converter for all the format you need.