Direction Fields for Differential Equations
This Demonstration lets you change two parameters in five typical differential equations. Observe the changes in the direction field and long-term behavior of the system. Notice the changes in both the lines of equilibrium and the direction of the field.
 W. E. Boyce and R. C. DiPrima, Elementary Differential Equations with Boundary Value Problems, 9th ed., New York: John Wiley and Sons, 2008.