FFMpeg

FFMpeg is a collection of open source libraries and utilities that all have the same purpose, which is to edit multimedia files, but primarily video files. The tools included in the pack allow you to record, convert and to playback audio and video files. One of the included utilities has the ability to convert and to resize videos instantly by using a high quality poly-phase filter. The converters and media players are compatible with hundreds of different codecs, so you will always be able to watch any video or listen to any song that you want.

FFMpeg uses a command line as its interface and isn't too hard for advanced users to utilize, since it fills in most of the code for you and you usually just have to specify a couple of parameters. However, it might be a little difficult for beginners to use. Fortunately, there are many programs that you can install that will give FFMpeg a graphical interface, such as AVANTI.