Simple Javascript implementation
http://codeflow.org/entries/2010/aug/28/integration-by-example-euler-vs-verlet-vs-runge-kutta/
Orbital
This one is promising, Code in python:
https://github.com/RazerM/orbital
Some of the provided examples:
# Create circular orbit with 90 minute period
orbit1 = KeplerianElements.with_period(90 * 60, body=earth)
plot(orbit1, title='Orbit 1')
# Create molniya orbit from period and eccentricity
from orbital.constants import earth_sidereal_day
molniya1 = KeplerianElements.with_period(
earth_sidereal_day / 2, e=0.741, i=radians(63.4), arg_pe=radians(270),
body=earth)
plot(molniya1, title='Molniya 1')
# Create circular orbit at altitude 300 km
orbit2 = KeplerianElements.with_altitude(300 * kilo, body=earth)
plot(orbit2, title='Orbit 2')
# Create molniya orbit by specifying altitude at perigee.
# This works because M0=0 (perigee).
molniya2 = KeplerianElements.with_altitude(
508 * kilo, e=0.741, i=radians(63.4), arg_pe=radians(270), body=earth)
plot(molniya2, title='Molniya 2')
Orbit Hopper
2D Godot game:
https://github.com/grzala/orbit_hopper/