Data mining

I need some data extracted from certain websites. The websites have lots of users. I want the names and email addresses of these users. SOme websites offer Names and email addresses but other websites dont give the users' information. I am not sure how to get data from those websites but I think it can be done through some script. There are approx 2000+ names that need to be extracted in the beginning but if all goes well this project could exceed to more than 500000 names. I am open to all suggestions on how to do this work.