MediaCoder will transcode or merge almost any
kind of video or audio formats and you can also use it to add subtitles to video files. It's not always that easy to
use, though. There is a lot of settings to take care of.
Personally, I use Handbrake and MKVmerge
(MKVToolNix) because they can keep the quality of the vid files pretty high but I'm afraid these are for the mkv (OR
mp4) format only so if you have any other video format like avi, it most probably won't work.
Also, I moved this to 'Computers & Internet' because it's not about music and movies as a form of entertainment.