Since PTA has blocked youtube.com, people in Pakistan are looking for ways to run the website. I would not go in to the moral issues of using the website.
Anyways, there are few websites through which you can watch the videos of youtube.com or any other blocked site.
Go to www.google.com and type "bypass proxy" in search
All the results will lead you to website which will bypass the restrictions implemented by PTA.
Some of them might not work, so just go to the next one.
Once these websites open, they will have again a search box where you will type www.youtube.com and click on Go.
Youtube.com will be opened with in that browser. You can watch as many videos as you want. So ENJOY!!!