Before I had ever used Matlab, I wrote something resembling it (in JavaScript, of all things). I used it at the time for my statistics class, and kept the code kicking around, but never got around to finishing everything that I had planned. (I had wanted to add many more statistics features). One of the factors was JavaScript's lack of support for overloading operators, which kind of stopped things. Also, there isn't really a convenient way to support complex numbers.
Today I made it (somewhat) cross-platform. Since this might be as complete as it will get, I'll call this the last version. Open the page and you can click a link to see a list of features and provided functions.
I still think this is a pretty cool project, and I am just now working on something similar in Python.