Initial development of the Search API, as well as of some others of my modules, was sponsored in various ways. I was also lucky enough that some companies were interested enough in some features or bug fixes to pay me for their development. However, most issues aren't backed by any sponsor and I have to work on them in my spare time. Since I now maintain almost ten modules, most of them with thousands of users, this becomes more and more unsustainable.
Therefore, if you like some of my modules, please consider contributing in one of these ways (or any other you can think of, of course).
Also, check out How to get involved on drupal.org, which largely applies to contributed projects, too.
If you have another idea on how to contribute, or need help with starting to contribute, please contact me.
No matter in which way, your help would be very much appreciated!
Help in the issue queue
My issue queue – containing all issues of my modules, as well as a few others I participated in. As you can see, it has currently (May 2013) over 700 issues in it. If you can code and are familiar with the code base of a module, writing patches (especially bug fixes) is of course a great way to help me here.
But there are also lots of other ways in which you can help: test patches (there are usually lots of issues blocked because no-one tests/reviews their patches), try to reproduce bugs, answer support requests or, for the loads of old bug reports, ask whether the problem still exists in the latest dev version of the module and close the issue if no-one answers within a couple of weeks.
The d.o handbook page Helping maintainers in the issue queues is also a good description of the possible ways you can help.
Help with documentation
Though I started to put up some documentation for the Search API in 2011, it is rather incomplete (e.g., documentation for developers is missing almost completely) and most other modules of mine aren't documented (apart from their READMEs) at all. Fixing bugs and other important coding tasks just take up too much time. So, if you know your way around the Search API (or other modules), contributing to the documentation would also be a good way to help lots of people, and perhaps also free me from some new support requests.
If you want to support my work with a donation, my PayPal account is firstname.lastname@example.org. I'm also on Gratipay. If you prefer other means of donation, please contact me. All donations will fund development for my Search API modules.
If you want me to work on a specific issue or project, please send a message along with your donation – however, there is naturally no guarantee I'll always be able to follow your suggestion.