Textbook solutions

Algebraic Topology — Allen Hatcher

Various solutions through chapter 3 (homology and some cohomology)

Basic Category Theory — Tom Leinster

Solutions to assorted exercises from the whole book

Sheaves in Geometry and Logic — MacLane and Moerdijk

Solutions to exercises and proofs of theorems in introductory topos theory

Algebraic Topology — William Massey

A few solutions from the chapters over fundamental group and covering space theory

Theory of Computation — Michael Sipser

Assorted solutions covering finite automata theory