There's going to be an interesting gap in the market if Google site search goes away. Algolia handles search elegantly for developers, but doesn't solve many of the problems (indexing, spidering, stemming, rendering, etc.) for the novice, in the way Google Site Search does.

However, my understanding is that Google CSE would basically replace site search and can be configured to operate identically, and isn't slated to disappear.

Algolia is one competitor, but there are many others which actually does the indexing through crawling, stemming and UI rendering - ours, Cludo.com being one of them.

Got to admit, Algolia is the only site search I've ever liked.

If I'm not mistaken, Google Site Search is targeted for shutdown by Apr 2018. They're probably refocusing their resources to other more lucrative product lines.