VLC may serve.

From The Best Hidden Features of VLC:

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.

VLC may serve.

From The Best Hidden Features of VLC:

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.