OggConvert is a simple, GUI-based video transcoder that outputs only to the free Theora and Dirac formats. It couldn't be any easier to use, and it's the quickest way to get a feel for the still-new Dirac codec. No need to tweak pages of arcane settings -- just drag, drop, and watch.
You can download OggConvert as source code, or as a prepackaged binary for Debian, Fedora, SUSE, or Ubuntu. The latest release is version 0.3. OggConvert is written in Python, and uses GStreamer to perform its media conversion work.