This is a must join. The difference between clickbank and commission junction is that clickbank is more downloadable and informational products where commission junction is more tangible products (ex: home depot, buy.com, best buy, dell).
You are going to want to check them out. But, keep in mind, you will need to have a website or a newsletter. They do not work with anyone who does not have a website or newsletter.
So, make sure you understand what you are trying to do. Affiliate marketing can be very lucrative, however, you need to be working with the right programs.