RKF45 is a FORTRAN90 library which implements the Watt and Shampine RKF45 ODE solver.
The RKF45 ODE solver is a Runge-Kutta-Fehlberg algorithm for solving an ordinary differential equation, with automatic error estimation using rules of order 4 and 5.
CWG_ODE is a FORTRAN77 library which contains three ODE solvers by C William Gear.
LSODI is a FORTRAN77 library which implements the Livermore Solver for Ordinary Differential Equations, Implicit case.
NMS is a FORTRAN90 library which includes a wide variety of numerical software.
ODE is a FORTRAN90 library which contain the Shampine and Allen ODE solver.
POLKING is a pair of MATLAB routines, DFIELD and PPLANE, for solving and displaying the solution of differential equations and their phase planes.
RKF45 is also available in a C++ version. and a FORTRAN77 version.
TEST_ODE is a FORTRAN90 library which contains routines which define some test problems for ODE solvers.
You can go up one level to the FORTRAN90 source codes.