The world may not have needed yet another calculator, but I wrote this while playing with the Spirit parsing library, and maybe some people will find it useful. PlainCalc is a buttonless, algebraic-entry, mathematical calculator with variables. To use PlainCalc, type an expression or assignment and press Return or Enter. Unlike most calculators I've seen, PlainCalc allows you to indicate multiplication by juxtaposition, e.g., you can write 2πr instead of 2*π*r.
For those using older OS versions, including OS 9, an earlier version of PlainCalc is available below.