I shared a while ago how the National Science Teachers Association has built a list of over 200,000 prospective members by using this technique. They basically exchange content for contact.
A friend of this blog from Convio recently shared with me their non-profit online benchmarking survey that highlights many online practices of non-profits including email list building. You can find out more about the study here.
One statistic that they shared is the medium size of email lists that have been developed by different categories of non-profits. Here is the chart from their report. 
A key step in building these files is to convert visitors to your website into registered users. Convio shares in their report that associations on average are converting 3.05% of visitors into registered users by getting them to volunteer their email address.
Do you have an active registration and email program with your organization?
 Quinn Donovan, Convio Analytics Manager , Vinay Bhagat, Founder and Chief Strategy Officer, Brian Hauf, VP Client Success Services, The Convio Online Marketing Nonprofit Benchmark Index™ Study, March 2009.