‘Curlew’ is a multimedia converter (GUI that uses ‘ffmpeg’, ‘avconv’ and ‘mencoder’ -- command-line based tools) for GNU/Linux. It is an easy to use application that lets you convert over to more than 117 formats, says the author.
You can choose between a lot of built in presets, preview files before encoding and change advanced encoding settings such as changing the audio/video, bitrate, channels, frequency, aspect ratio, FPS, resolution, enable 2-pass encoding, crop, split (based on duration), change various subtitle based settings, shutdown the PC when done etc.
The recently released 0.1.21 version has brought a few new useful features such as the ability to view advanced file information (codecs, bitrates, aspect ratio, width & height, FPS etc.), new icon set, the ‘Duration’ of the files on the list, a new menu entry for opening the location of the files on the list, are among the major changes.
If interested you can install it on Ubuntu 13.10 (I tested the latest version on 13.10 only) from this page. Make sure to download the latest ‘Deb’ package (supports both 32-bit and 64-bit instructions), and once the download completes, double click on it and follow the instructions on Ubuntu Software Center for installing it.
Pre-built packages are available for other distributions also. For obtaining those files, please refer to this page.