There are many niches where marketers hold giveaway events. This is pretty much a one-stop shop where people in your niche can grab free offers that they are interested in. People will sign up for your list in return for getting your free product.
Putting your opt-in box on one of these giveaway websites is another way to get increased exposure to those who might not have otherwise seen what you have to offer.
Now, I will admit that most of the leads you get from these giveaway offers are of pretty low quality. But there are exceptions to the rule. You'll have to try this out to see what sort of results you may get.