I've occasionally seen sources of algorithms/data structures posted on HN, but they've mostly been fairly low quality. This source was created/maintained for the International Collegiate Programming Competition, which has a 25 page limit.

Among others, the last 3 MIT ICPC teams have used a variant of this notebook.

Some neat things in this notebook:

The fastest 64 bit mod mul implementation I've seen (uses doubles and proven correct): https://github.com/kth-competitive-programming/kactl/blob/master/content/number-theory/ModMulLL.h

A fast/short implementation of FFT: https://github.com/kth-competitive-programming/kactl/blob/master/content/numerical/FastFourierTransform.h

A bump allocator: https://github.com/kth-competitive-programming/kactl/blob/master/content/various/BumpAllocator.h