There are certainly the ones I use them most and probably couldn’t live without.
I have placed them in no particular order.
Youtube - Not only is this site full of endless hours of entertainment. Its helped many talented people be discovered. The site is so popular it even has its own TV show called RudeTube. Which shows the funniest and silliest videos to feature on the site.
2. Facebook - Nearly the entire world uses this site for one thing or another. Staying in touch with friends, playing games, Promoting a business or product. Its so popular its almost sickening. There was site a movie about the site called The Social Network, which wasn’t so popular.
Twitter - It took me a long time to understand the appeal of this site but now I finally know. Its a great way to share your random thoughts with people all over the world you won’t be able to reach otherwise. Its also a very useful promotional tool.
4. Google - How could I not put this of the list. This site is the head of the internet and has the answer to almost anything. Pure genius.
5.Yahoo Answers - On the rare occasion Google does not have the answer I turn to this site. I had found this to be invaluable when I’ve needed to do research for some of my articles.

Comments
My own list would include Wikipedia - an indespensible source of information at one's fingertips.