Email doesn't need postage, right? Is that old hoax about the postal service charging per message finally coming true? Not really. But PostageApp and and their API may be a service worth paying for if you want to send email from your application.
In many web programming languages, such as PHP, it's already easy to send email. However, making a user wait while the code sends out a dozen emails to their friends means your site seems slow. Plus, your email may be plain text and boring. And who knows whether your host's server has been blacklisted by the actions of your more shady developer brethren.
Plus, unless sending out emails is a core part of your application, it makes more sense to let someone else do it for you. As PostageApp says, let "your app do what it does best... be awesome."
Right now the service is in an open beta, though the company has announced pricing plans are in the works. There are some limits during the beta period, especially if you're using PostageApp's SMTP server. But there's certainly enough functionality to tell whether it will work for you. And if you program in Ruby, there's a how-to video (embedded below) to show you the way.
We're seeing more of these sorts of services, which plug into a very specific feature that many apps have. These services save developers time, which is something many are willing to pay for. Look for PostageApp and its ilk to find plenty of interest, as building apps becomes more about focusing on your core competencies, rather than trudging through those annoying-but-necessary elements.