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.
A blog highlights your technical ability and showcases your ability to write blog posts. Your blog can be about different topics than those you write about for your clients. In fact, it should be on a topic that interests you. Visitors will see that you can not only write, but you can also build an online community. A good blog has the potential to earn you many referrals for more clients.
Are you a professional in a field that can help answer questions for people looking for your expertise? Websites like JustAnswer and LivePerson match you up with people looking for answers to technical or professional questions. You can make money online by simply answering these questions and providing the right information to people based on their individualistic circumstances.