Ever tried to play a video and you got an error saying this file type isn't supported? Or you need to edit video in Vegas or Premier but it wouldn't recognize it? What about transferring your movies to your iPhone or Android in a way that it can actually read it? Then you should check out Handbrake.
Handbrake is a very powerful tool for doing video conversion. It's also fast, easy to use, and doesn't contain a bunch of fluff to convince you that it has more features then it needs. The presets make using this app dead simple. It will also backup your DVD's or Blu-Rays giving you the option to select the chapters and audio tracks. There are many more features available that I have no idea how to use but I'm sure is useful to someone a lot smarter then me.
I use Handbrake a lot for converting videos made with my screen capture software to something Sony Vegas can read. For some reason it doesn't like the AVI files that Camstudio outputs. So I use Handbrake and convert them into MP4 files. Works effortlessly.
This software is free and open source. It's also available on Windows, Linux, and Macintosh. There isn't any spyware or malware during the install either. I avoid apps that install crap on your computer like the plague no matter how good they are. Anyways go to the site and check it out for yourself.
The open source video transcoder