This page is mostly a digest of the book "Fundamentals of Astrodynamics" by Bate, Mueller and White (known as the "BMW" after the authors). This page is useful as a reference but not really good for learning material. I find the Wikipedia pages on this subject a bit shallow too (one of these days I'll contribute improvements to the Wikipedia pages), so if you intend to learn this stuff, obtaining a book is the only viable option (if you know of other good online resources, please share with us. I don't). The 1st edition BMW book from 1971 is available for dirt cheap (15-20 USD), the 2nd edition came out just a month ago but it's poorly available (I'm still waiting for my copy from Amazon).

There's a few more pages (incl. interplanetary mission planning) if you go up one level: http://www.braeunig.us/space/

Finally, here's a project of mine that implements a bunch of algorithms for two body orbital mechanics: https://github.com/rikusalminen/twobody

These algorithms are mostly based on the BMW book as well as a bunch of research papers from the 1960s to 1990s. Look around in different branches for some of the more advanced algorithms (including a search for closest approaches and sphere of influence transitions like Kerbal Space Program and a boundary value problem solver for interplanetary travel), they're still work in progress.

One day I'll turn this into a mission planner software for space simulator games (or some kind of game, not sure yet). If you happen to need some orbital mechanics algorithms, take a look of the above and I'm happy to answer any questions or help with this.