Online Converter is a great resource for anyone. The free site can convert file formats for audio, video, documents, archives, images, ebooks, and more. The site can convert almost any format to another. This is a great resource when dealing with multiple computer systems and software (Apple vs. PC, Word vs. Writer, etc.).
It is easy to use and works great.
VLC - Awesome free video and audio player (and converter)
Lots of PDF resources - print, markup, convert and more
Media Converter - fast free online audio and video converter