Simple math of everything
But for people who can read calculus, and sometimes just plain algebra, the drop-dead basic mathematics of a field may not take that long to learn. And it's likely to change your outlook on life more than the math-free popularizations or the highly technical math.—Eliezer Yudkowsky, The Simple Math of Everything
Relates the speedup of a sub-task to the resulting speedup of the whole. Trivially true, but often needed to knock down false intuition.
Used to abstract away units and fixed overhead when analyzing resource usage.
Deterministic Finite State Automata
Traditional square one of theoretical computer science, with many practical applications.
The Pumping Lemma for Regular Languages
Illustrates many recurring themes. Understanding the proof and usage of the pumping lemma will help you understand and apply more famous, advanced results (e.g. anything involving Turing Machines).