RAB: This presentation

RAB: traditional printed monograph on Combinatorial Designs (CUP)

RAB: A Second Course on Linear Algebra (open-source now)

Judson: Abstract Algebra: Theory and Applications (beta: AY 2014-15)

Judson, Hitchman: Ordinary Differential Equations Textbook (in-progress)

RAB, Godsil: Explorations in Algebraic Graph Theory with Sage

Others at initial stages