I've been working on this for quite some time, it's been a pet project of mine since I finished my last web-based school project. My goal is to turn it into a fully fledged school/homework help forum, but for now it's an online calculator and graphing app with a toy BBCode editor. I have some screenshots, but no running server to actually let anyone play with it.
These few are from the original incarnation:
On a different note, the BBCode translation works fairly well, though needs a tweak on how it treats whitespace. the following two images show the test that I have working, it auto-updates when the text area is chaned.
Building Parsers With Java by Steve Metsker. I worked off the books Arithmetic Parser chapter and classes, and edited it heavily to change it from directly calculating values, to generating a re-usable parse tree, added the ability to store and use variable names, recognize graphs and recognize mathematical functions such as sine, cosine, tangent, logarithms, etc.
I have some of the forum related code in place, it's a matter of generating test data (forums, threads, users), and getting the code to access and display it all, so far it only displays one or two of the approximately 8-10 different things.