A bloom filter is a probabilistic version of set. Set contain elements and can tell you in O(1) time complexity and O(N) space whether or not it contains that element. A bloom filter can tell you whether it probably contains an element, but in O(1) time and O(1) space!

A prefix trie is a data structure that allows you to quickly look up a string by its prefix and also find strings that share a common prefix.

A ring buffer is more of a nifty way to use a normal array, but in a clever way that makes it optimized for data streaming.