My top 3 traffic sources for list building:

#1 JV-Traffic

Joint ventures - this is a win-win deal for you, you simply partner up with someone within your particular niche or subject matter. You could do cross-promotions with other online businesses within your marketplace and benefit from each other's list, leads and customers. Either way JVs are brilliant, but when first starting out beware JVs will be hard to come by.

#2 Facebook Advertising