Functional equations - Alex Song
Good algebra problems - Alex Song
Inequalities - Alex Song
Linear algebra - Edgar Wang
Polynomials - Robert Garbary
Sequences - Robert Garbary
A few geometry problems - Victor Rong
Miquel Point - Victor Rong
Incircles - Victor Rong
Projective geometry - Victor Rong
Combinatorics - Dani Spivak
Generating functions - Dani Spivak
Induction - Bill Huang
Invariants & colourings - Robert Garbary
Probabilistic method - Dani Spivak
Bounding in number theory - Alex Song
Diophantine equations - Alex Song
Cool ISL problems - Dani Spivak