I've recently been experimenting w/ facebook fan pages and aweber forms to build lists. If you use the right keywords when naming your fan page, and use an enticing image, you can build a list in no time. Now like in any list building you do, you have to of course use the proper keywords and enticements to get a list that will respond, so keep that in mind. Then if you also use some of the viral techniques others have talked about here on the forum, you could have a very large and responding list in no time.
Here's an example of list building for the farmville niche:
FarmVille - Free No Wither Crops are Here... Free No-Wither Crops Here... | Facebook
And here's instructions on how to put a form on your facebook fan page:
Edit - thought I should mention, aweber is not free, so technically it's not completely free. But for the most part, the technique is free.