I've just finished implementing a new search engine for this site. Obviously, with it using a static site generator, searching a relational database isn't an option. For a long while I'd just been getting by with Google's site-specific search, which worked, but meant leaving the site to view the search results.
Now, I've implemented a client-side search system using Lunr.js. It wasn't too time consuming, and as the index is generated with the rest of the site and loaded with the page, the response is almost instantaneous. I may write a future blog post on how to integrate Lunr.js with your site, as it's very handy and is an ideal solution for implementing search on a static site.