[**VLC**](http://www.videolan.org/vlc/index.html) may serve.

From [The Best Hidden Features of VLC](http://lifehacker.com/the-best-hidden-features-of-vlc-1654434241): 

> 1. Find a video on YouTube and copy the URL from the address bar. 
> 2. In VLC, head to Media > Open Network Stream. 
> 3. Paste the YouTube link in the box and click Play. 
> 4. Under Tools, click Codec Information. 
> 5. In the box that says Location, right-click the block of text and click Select All. Copy this text to your clipboard. 
> 6. Go back to your browser and paste the link in the address bar. This will open the source file directly on YouTube's servers. 
> 7. Right-click the video as it plays and select Save Video As.

That'll give you an `mp4` file. VLC can also convert some file formats.