Besides funding non-profits and high-impact for-profits, altruists can pursue less common funding models.
One example of an unusual funding model is the use of certificates of impact. On this model, altruistic work receives some or all of its funding after completion rather than beforehand. Once an individual or organization completes work with a positive social impact, they can apply for a certificate of impact. They can then sell this certificate to another organization or individual. Following the sale, the new certificate holder can claim credit for the impact of the project, and the organization that carried out the project must acknowledge that they have sold its impact.
This scheme has been proposed and tested by some members of the effective altruism community. They argue that the scheme is better than funding incomplete projects, because it allows for payments by results rather than by effort, which better incentivizes those running the project to do a good job. They also argue that awarding certificates of impact might be simpler and clearer than pre-funding projects.
A less formal mechanism for rewarding socially useful work after the fact are prizes and honours for doing good work, such as the Nobel prizes. A more formal mechanism is a social impact bond, in which investors provide up-front funding for services and a donor or government funder commits to paying them back (plus a risk premium) depending on the social good achieved by the project.
Brunt, Liam, Josh Lerner & Tom Nicholas. 2012. Inducement prizes and innovation. Journal of industrial economics 60(4): 657-696.
Impact Purchase. 2015. Why Certificates?
An explanation of the benefits of certificates of impact.
Kuhn, Ben. 2015. Impact Purchase.
A good intuitive description of the idea of impact purchase.