The New York Times released PourOver, a library that lets you do database-like things client-side, so that (1) you, the developer, can worry less about database optimization and server loads and (2) users get a more responsive, faster experience.
PourOver is built around the ideal of simple queries that can be arbitrarily composed with each other, without having to recalculate their results. You can union, intersect, and difference queries. PourOver will remember how your queries were constructed and can smartly update them when items are added or modified. You also get useful features like collections that buffer their information periodically, views that page and cache, fast sorting, and much, much more.
Also: How great is it that The New York Times is now getting into the habit of releasing code?