You can find the traffic of a popular website (and compare it to another site) by entering the URL into compete.com. Or quantcast. This data is far more accurate than the charts Alexa offers, because most of the sites being measured cooperate. I'm pretty proud of Squidoo hitting the top 100 sites in the US.
You can see the referrals and traffic to an individual bitly twitter URL by copying the URL and adding + sign to it. For example, if you see something like this in a tweet: http://bit.ly/870Ry9 just copy it and paste it with the plus and you'll see http://bit.ly/870Ry9+. I think that's pretty neat. You can also track top retweets on an hourly or daily basis.
Many sites also publish how many subscribers they have. The "K" stands for thousands.