Here is about the most basic possible matlab file for plotting the solution of an initial value problem for a first-order scalar ODE: ode.m
Here is a basic matlab file for plotting direction fields of a first-order scalar ODE: dir.m
A sophisticated MATLAB file for plotting direction-fields, written by John C. Polking, is available for download here.
This file plots the solution of an IVP for a second order linear ODE ode2.m