Links
Maths

Notes on just on about everything by Karl Dahlke

The University of Pennsylvania page of the late Herbert Wilf, generatingfunctionology along with some other of his books are provided free of charge here

“What Is” articles by Nick Higham  a lot of stuff on linear algebra and matrix analysis

Formulae Grimoire by Fredrik Johansson

An Infinitely Large Napkin by Evan Chen
Programming

Character Sets by Joel Spolsky

Isolation in DBs by Martin Kleppmann
Question Sources

Rosalind: a platform for learning bioinformatics through computational problem solving

IBM monthly puzzles (Ponder This)

Monthly Maths Riddles by Michael Brand (Inactive)

Cryptohack: learn modern cryptography