Brainstorming and Generating Business Ideas

There's pretty much a website for every hobby, interest, topic, etc., but I think it's helpful (and obvious) to take a look at what websites are popular, and it might give you a better sense of what your next domain name or website development will be:

Alexa:
http://www.alexa.com/

Alexa even has the top 1,000,000 sites available daily:
http://s3.amazonaws.com/alexa-static/top-1m.csv.zip

Ant:
http://www.ant.com/top

Google Trends:
http://www.google.com/trends