Differential Equations and Oscillations
Many problems in physics are described by differential equations. As a complete discussion of differential equations is beyond the scope of this chapter we will deal only with linear first and second order ordinary differential equations.
http://www.physics.orst.edu/~rubin/nacphy/ComPhys/DIFFEQ/mydif2/

Differential Forms in Electromagnetic Theory
BYU Electromagnetic Field Theory and Differential Forms Research Group
http://www.ee.byu.edu/ee/forms/forms-home.html

Topics in Integral and Differential Calculus
An explanation and demonstration of the differential formulas, including an applet.
http://www.ma.utexas.edu/users/kawasaki/mathPages.dir/index.html

Exterior Differential Calculus
Mathematica Packages for Exterior Differential Calculus
http://www.inp.demokritos.gr/~sbonano/EDC/ExteriorDifferentialCalculus.html

Differential Geometry
Several books by Peter W. Michor et al. including "Foundations of Differential Geometry", "Natural operations in differential geometry" (corrected version), "Transformation Groups", and "Gauge theory for fiber bundles" plus papers by the author in postscript.
http://www.mat.univie.ac.at/~michor/listpubl.html

Easy-Fit
Used to identify parameters in explicit model functions, dynamic systems of equations (steady-state), Laplace transforms, ordinary differential equations, differential algebraic equations and one-dimensional partial differential equations. Proceeding from given experimental data. For example, observation times and measurements, the minimum least squares distances of measured data from a fitting criterion are computed, that may depend on the solution of the dynamic system.
http://www.uni-bayreuth.de/departments/math/~kschittkowski/easy_fit.htm

Differential Evolution
Fortran 90 code implementing the Differential Evolution method, a simple but powerful population-based, stochastic function minimizer.
http://www.icsi.berkeley.edu/~storn/DE_FORTRAN90.f90

Differential Equations and Mathematica
An online companion for differential equations contains demo notebooks, packages, and links.
http://www.math.armstrong.edu/faculty/hollis/mmade

Stochastic Differential Equation Software
Fortran 77 code for numerical solution of stochastic differential equations such as those which arise in Black-Scholes theory and its generalizations.
http://www.isalgorithms.ca/examples.html

Electronic Journal of Qualitative Differential Equations
Free research papers on differential equations.
http://www.math.u-szeged.hu/ejqtde/