Yamanote Line

I am traveling to Tokyo for 5 days. Is there a one day pass I can purchase to use the Yamanote Line unlimited all day.

Yes, there is the Tokunai Pass for 730 yen:
http://www.japan-guide.com/e/e2017.html

You would have to use the Yamanote Line about 4-5 times or more in order for the pass to pay off. In other words, it is not unlikely that regular tickets or a prepaid card would come cheaper.

If you do plan to use JR a lot, don't limit yourself to the Yamanote Line alone (I know it is easy for many because it loops), but also utilize the other JR lines such as the Chuo and Chuo-Sobu lines as well.

For example I had a friend who rode the Yamanote all the way from Shinjuku to Akihabara, however, you can jump on the Chuo-Sobu Lines and do it in half the amount of time.

http://www.japan-guide.com/e/e2017.html