OC-Web
Over recent years, I've been teaching Astronomy 310 (Stellar Astrophysics) using the textbook An Introduction to Modern Stellar Astrophysics (Dale Ostlie and Bradley Carroll; 2nd edn.; ISBN 978-0805303483; available from Amazon.com). There are a number of end-of-chapter questions in the book which rely on computer codes available (as source or executables) from the book's companion website. To make these codes more accessible to students who may not own a computer (although are still able to get on-line via public computers), this page collects together web-based interfaces that I've developed for some of the codes.
Orbit
From O&C: "Orbit is a computer program designed to calculate the position of a planet orbiting a massive star (or, alternatively, the orbit of the reduced mass about the center of mass of the system). The program is based on Kepler's laws of planetary motion as derived in Chapter 2."
TwoStars
From O&C: "TwoStars is designed to provide position, radial-velocity, and binary light curve information that can be used to determine masses (m1 and m2 from determination of the semimajor axes and periods of the orbits), radii (R1 and R2 by measuring eclipse times), effective temperature ratios (from the relative depths of the primary and secondary minima), limb darkening, orbital eccentricity (e), orbital inclination (i), and orientation of periastron (φ)."