Alexa.com is a gold mine.
You can use it to see how popular a website is. You can use it to see how popular your site became and if you want to buy advertising on a site you can use Alexa rankings to see if it is worth it.
The Alexa will show very useful graphs like:
- daily reach
- page views
Suing Alexa you can find the most popular sites in the world. Just visit http://www.alexa.com/site/ds/top_500.