As @good-karma mentioned, if the application a developer makes allows users to post, then the post rewards beneficiaries allows you to claim a percentage of what the post author's payout would be. This feature hasn't been that widely used yet, and may have an impact on adoption.

There's a few other ways as well:

• Becoming a Witness: Developers who stick around and get deeply involved in the steem ecosystem can choose to campaign to become a witness. Witnesses run witness nodes, which generate blocks on the network and earn a rewards. Being a witness is a lot more than just being a developer though; you also need to be proficient with systems administration and pay for your own servers.
• Posting updates about your projects: Did you add a new image uploader into your app? Did you release a new project? Did something awesome happen because you did something cool? Write up a post about it. Semi-regular post rewards can help make the earn some money.
• Creating a crowdfunding address and ask for donations: A lot of developers do this, and there's a lot of deals that happen if you network. Some of the whales within the community sponsor projects and help keep 3rd party developers fat and warm.

I'm sure I'm missing a few other ways as well!

As a developer who does earn money by developing apps for the steem blockchain, I can say that it's a pretty unique system. It just takes a while to figure out and some networking with others in the community.

Hope that helps!