Probabilistic algorithms

https://www.safaribooksonline.com/oriole/probabilistic-data-structures-in-python

Set Cardinality - HyperLogLog

Set Membership - With False positives but no False negatives - Bloom Filter

Set similarity (document similarity) - MinHash with Jaccard

Frequency Summaries (Leaderboards in games) - Count-min sketch

Streaming Quantiles - stream of data where you have to aggregate metrics without stopping - T-Digest (like min/max/percentile)