I'm limiting my choices to open source with good explanatory write-ups or presentations.

Graphite for "simple" time series storage:

https://www.aosabook.org/en/graphite.html

HLL datatype (PFCOUNT, PFADD, PFMERGE) in Redis for counting unique items in a set:

http://antirez.com/news/75

Data structures and algorithms that make Lucene & Elasticsearch fast (video):

https://youtu.be/eQ-rXP-D80U