First, let me say that I have been using MathCAD since 1986 and it is a great program.  The behavior of PTC, the owners of MathCAd have been actively driving loyal customers away.
I found SMath a few minutes ago, downloaded it, and tried a few simple calculations.  This is truly a masterful undertaking.  I have one problem, however and that is with the SMath community.
I see many comments and requests for features but little activity.  This is not the problem of the developer.  Look at the donation banner at the top.  Since, January 2010, only $1321 have been donated.  Dig into your pockets and send in a few dollars!  I did, and I've only been a member for 30 minutes.