But as the time passed on... thanks to some hard work of many developers now we have not just players but a whole bunch of multimedia frameworks (Phonon, Gstreamer, etc) which are sometimes decodes the proprietary codes, even efficiently than their official decoders!.
Okay, before I go off topic let me start the post right away :D. Transcoder is a GTK+ built GUI front-end that uses the ffmpeg multimedia library that lets you convert a huge list of multimedia files (both proprietary and open-source) with ease!.
*. A simple GUI that looks clean and professional.
*. Supports multiple codecs (both audio and video) such as: Theora, H.264, Xvid, MPEG1/2, FLV, Vp8, AAC, MP3, Ogg Vorbis, Amr, direct stream copy (means copy the audio/video content to a new container without encoding) etc.
*. Change video resolutions.
*. Change video and audio bi-trate according to your preference.
*. Change frame-rate and aspect ratio.
*. Audio volume Gain, sample rate and channels.
*. Embed subtitles.
*. Add/manage a lit of multimedia files with ease.
When you convert a file, Transcoder also gives you an option called "slower" which puts the converting process in a bit low priority so it'll still convert but you'll still be able to use other apps since the "standard" way of converting consumes almost 100% of your CPU thus slows down the PC/Laptop.
You can install Transcoder in Ubuntu 11.04 Natty Narwhal (might also work in 10.10 and 10.04, haven't tested it though) by first downloading the ".deb" package (supports both 32/64 bit platforms) from this Transcoder's home page.
Once the download completes, just double click on the .deb file which should open-up the Ubuntu software center. Then simply follow on screen instructions and USC will take care the rest for you :).