Wikipedia writes: Maxima is based on a 1982 version of Macsyma
Perhaps because it has such a long history and because this was published in a Lisp website, Maxima was considered known.
>> This looks like an open source alternative to Mathematica/MathLab/Maple running in the browser.
Yes. If you're on Linux try out wxMaxima as a local app. I'm not sure how to get the Windows version of wxMaxima since you end up in some weird sourceforge hell trying to get an installer.
Please correct me if I'm off...
As there's no project description that I quickly found, I came to this conclusion myself.