Sites

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

Pseudo-Differential Operators (Popularity: )
http://arxiv.org/abs/math.AP/9906155
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 ...

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

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

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

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