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.
If you have a propensity for writing and you can slay with your prose, consider writing an ebook. While the market has certainly become saturated as of late, books that help teach people about a technical topic still sell extremely well. This is a great source of passive income but does require a large amount of effort at the outset before any money is generated.
Has anyone ever told you you have a voice for radio? Are you great at creating original characters with just your voice? There are tons of people looking to pay for quality voice overs for their corporate videos, animation series, or educational videos. Check out Fiverr and UpWork or create a profile on a specialized site like Voices.com or The Voice Realm to get started making money online doing voice overs.