YouTube Portable App

doing specifically this?

I might have mistaken something, but the videos are mostly downloaded via a browser, at least this is what I thought it is supposed to be.
So as a browser, firefox should be more then enought for that.

But there are medial players like the VLC under the applications list on this site.

You could try the Download Helper addon from http://www.downloadhelper.net/

I've not tried it portably, but it works great with YouTube and most other flash video sites.

There is something portable but its not in Portable Apps Format, but that doesnt matter... Its portable by default meaning after you install it copy and paste the program file in you C/: drive and uninstall it and there you go.

Here's a link: I think >_>
www.vdownloader.es/

It works by inputing the web adress or browses selected site on a built in search tool. So you can download it, Convert it to your desireble format too, and done. When using the search tool you don't need to load the video. Atleast thats for me.