MJ is an awesome game. There are many ways to explain it, with many different variations on the scoring rules, which have usually evolved per region in Asia, and more recently all over the world, but the essence of the game is "starting with a collection of tiles, exchange one tile per turn in order to get sets of tiles that are worth points". Then win with those points.
A typical wikipedia explanation of the game can be found here.
I typically play using the somewhat complex Chinese Classical rules (though I will play using the severely simpler Cantonese rules when in vancouver because everyone I know there only plays Cantonese), and in order to keep score I wrote a simple PHP file that allows me and the people I play with to keep score while we play, rather than having to calculate scores by hand at the end of each round.
One of my friends recommended I charge money for people to use it, which is in itself a decent idea but I have no idea how to go about styling it so that it's worth paying money for. For now, I'll go by donation: you can use it, if you use it a lot just donate a little money to nihongoresources (even something less than $5 is very welcome).
click here to go to the Chinese Classical scoring page.
I'm also every now and then working on an AI player algorithm for MJ. Right now I have an old version that works lying around somewhere, and a new version that doesn't work yet. So when it does, I'll be adding a bit more information to this page