1) The Cobb-Douglas Production Function
Not even micro-founded in its traditional presentation, but useful nonetheless.
2) Liquidity constraints
This is like being broke, but more complicated.
3) The Hodrick-Prescott Filter
Useful when you need to make messy real-world data confirm to a nice theory.
4) Dynamic Stochastic General Equilibrium
Eggerston (2008) proves that with sufficient application of ingenuity, you can make a DSGE model to prove anything.
5) Ricardo-De Viti-Barro equivalence theorem
Sure doesn't seem true, but a cool result for explaining to politicians why they're wrong about stuff.
6) The IS-LM Model
A macroeconomic model so simple even an undergraduate can understand it.
7) Michael Woodford's textbook
Next time some BA-wielding journalist wants to opine about monetary policy, ask them if they've even read Interest and Prices.
This has nothing to do with the painter.
9) The Hecksher-Ohlin theorem
Resource endowments matter, duh.