VLC Media Player is an open-source (i.e. free and legal) media player that is actually, in my opinion, better and more versatile than the likes of Windows Media Player or Quicktime.
It also has some great extra features one of which we will visit now. To convert files, just go to FILE then select CONVERT/STREAM
The next window, shown here will allow you to select the file you wish to convert (DROP MEDIA HERE), choose the format you wish to convert to (press CUSTOMIZE in the CHOOSE PROFILE section) then select a name for the new file you wish to create (in the CHOOSE DESTINATION section). And you’re done! It really is that simple! The conversion process itself may take lots of minutes depending on how big the file you are converting from and to is, and how powerful your computer is. You can monitor the conversion progress on the timeline of the player itself.
It is free to download and can be found here – Visit the VLC Media Download page.