Fractional Cascading. A simple and very cool way to speed up searching for the same key in multiple lists. Instead of K binary searches taking time Klog(N), you can do it in log(N) time without using asymptomatically more space.

https://en.m.wikipedia.org/wiki/Fractional_cascading

I wrote a simple demo in Rust a while back to help myself learn the language.

https://github.com/mgraczyk/fractional_cascading

I also think pairing heaps are neat.