Here is the answer to your question:
1. Technorati.com: This site is directory for all types of blog. You can submit your blog to this directory and as it is very strictly moderated you have to be very good in blogging and writing. This is highly appreciated directory of blogs which helps you to increase your rankings as well as no. of visitors.
2. Stumbleupon.com: It is site which redirects you to another sites depending upon interests you set while creating your account. When you log in it will direct you to sites which have more new and important content. It will also gives you permission to vote in form of 'Thumbs Up/Thumbs down, so it will become more interesting as you go on using it.
3. Reddit.com:This website is source to latest and most famous content on the internet. You can set your interest here also and when you log in to your account it will show latest content according to your interests. Reddit is short form for 'I read it on'.
4. Pininterest.com: It is website to store and share the images and videos to interesting and important webpages to us. We can also share these webpages with our family and friends. It is place to store links to related images and videos in one place to reference them for future.
All these websites are designed to give the user ease of use and ease store the information interesting to him. These websites plays an important role in SEO, as if your page is ranked high in these sites then you will definitely get more visitors to your site.
According to me these all sites serves different purpose, so it will not be fair to decide best and worst among them. You can choose the best site depending on your interests. Fo ex. if you want to share and store links to images and videos then pininterest will be best for you.
Sachin R. Kukale