Selene is a GTK3/Vala media converter tool developed by Tony George (who's also behind Conky Manager), which "aims to provide a simple GUI for converting files to popular formats along with powerful command-line options for automated/unattended encoding".
Besides supporting most popular audio/video formats, Selene supports 2 types of presets: JSON presets (which determine the audio/video format, codecs, bitrate, quality and so on) as well as Bash script presets which can be used for converting files using any command line utility, useful for integrating tools that aren't directly supported by Selene, as well as for automated / unattended encoding.
It's also important to note that Selene can also crop / resize / resample videos or embed subtitles.
The application is under constant development, with the latest version (2.4) being released about 10 days ago with SOX Audio Processing support, an option to check for missing encoders (see screenshot below) and bug fixes.
Selene - encoders status
Selene Presets - filter options
Selene Media Encoder features:
Encode videos to MKV/MP4/OGV/WEBM formats;
Encode music to MP3/AAC/OGG/OPUS/FLAC/WAV formats;
Option to pause/resume encoding;
Option to run in background and shutdown PC after encoding;
Option for SOX Audio Processing;
Preview file before converting;
Bash scripts can be written to control the encoding process;
Command line interface for unattended/automated encoding (run "selene --help" for a list of available commands);
Support for copying audio tags (Artist, Album, etc) to the the output file;
Option to check for missing encoders.
Update January 26th, 2015: Selene has received VP9 and H.265/HEVC encoding support. Note that You need to have ffmpeg/avconv compiled with VP8/VP9 support to be able to use the new VP9 encoder (Ubuntu 14.04 and above should have this enabled by default).
Install Selene Media Encoder in Ubuntu or Linux Mint
Ubuntu / Linux Mint (and derivatives) users can install the latest Selene Media Encoder by using its official PPA. Add the PPA and install Selene using the following commands: