First of all, whats the difference between list builders and let's say a safe list. well, they are both double opt in lists of people that want to advertise to each other but the real difference is in the amount of emails that you get from each.
Generally speaking, the safe-lists will virtually bury you with advertisements whereas the list builders limit the number of emails that you will get in a day.
The other thing is, you are actually getting peoples names and email addresses that you can market to. These people are the ones that are your referrals in the list builders that you belong to. With safe-lists you never get anyone elses contact information to keep for your own.
Now, these new list builder programs have arrived on the scene with some real momentum and are becoming almost as popular as the social networks.
Most have a free membership that is pretty restrictive, however if you do some research and read their websites to see what your options are you will find some really big differences.
Some of the things you should look for in these list buildersystems is whether or not the free members can earn referral commissions as well as being able to send emails to the membership on a limited basis.
Another thing to pay close attention to is the compensation plans for the upgraded members along with the pricing structures that are in place. These two things vary widely amongst the different companies.
All in all I would recommend joining a few list builders as a free member so that you are able to evaluate each one. Then when you find the one that you think will work the best for you, upgrade in it to receive all the benefit from the pro position.
I must also mention that most pay plans work best if you are able to join during the initial launch right when they open. This will help with the commission structure in the programs heaviest growth period.