With smartphones gaining popularity, the demand for their applications, or apps as they are popularly known, has also shot up. There are over 3 million apps for the iPhone and over a million apps in Google's Android market. Most of these are selling like hot cakes. Developing and selling your own smartphone app is becoming a lucrative way to make money on the Internet. Apps cost virtually nothing to develop and entail no storage or shipping costs. So they enjoy the best profit margins.
If you're serious about making money online, start a blog. Blogging is one of the easiest and most sustainable income sources. As long as the blog is setup the right way, in the right niche, with the right content targeted at the right audience, and the offer is complementary to the content, you could make a tremendous amount of passive income from a blog.
One of the cool things about Google AdSense is that it's so easy to get set up. If you have a blog or website, you can sign up for a free Google AdSense Account. From there, Google will give you a unique code that you will paste onto your website. Google takes it from there, tracking your page views, traffic, and earnings on your behalf. There is no upkeep or maintenance to get this thing going, which makes it a no-brainer if you have a website already.