Avidemus is a useful open source tool for tinkering with your video files. You can do some pretty crazy things with it like changing framerate during a video!
Did you know you can make videofiles that change Framerate?
I first thought this might be possible when ---- it's fixed now, but it used to be that you'd export an MKV file from Blender, bring it into the VSE, and it would come out entirely the wrong length....
Not strictly Blender related, but very much about open-source video software - and kinda interesting.
(the glitchy effect is intentional)