Differential Equations with Boundary Value Problems
   By S. Hollis (Prentice Hall, 2002). Overview of contents, errata, software and link to purchasing page.
   http://www.math.armstrong.edu/faculty/hollis/dewbvp

Green's Functions
   Lecture notes introducing this technique for solving first and second order differential equations. Scanned (GIF) by Joceline Lega.
   http://math.arizona.edu/~lega/583/Fall98/lectnotes/GF1.html

Pseudo-Differential Operators
   These lecture notes cover a first year graduate course that was given on pseudo-differential operators. The calculus on manifolds is developed and applied to prove propagation of singularities and the Hodge decomposition theorem.
   http://arxiv.org/abs/math.AP/9906155

Series Solutions to Differential Equations
   A set of downloadable lectures.
   http://www.math.hawaii.edu/~lee/calculus/#Series-Sol

Vibrating Drumhead
   A set of Mathematica packages which outline solutions of the wave equation in various coordinates.
   http://www.ma.iup.edu/projects/CalcDEMma/drum/drum.html

Waves and Oscillations
   Teaching resources developed by Frances Goldman, University of Glasgow.
   http://www.maths.gla.ac.uk/~fhg/waves/