Calcute is good, free, small, expression calculator: http://calcute.com/details.html
www.google.com also has good, free, built-in expression calculator.
I used Omax and Solway’s expression calculators in the past. Omax is obsolete. Solway has factorials, but otherwise it is less refined than Calcute.
I don’t mean to come of as overly harsh here, but this trend of posting stuff that is (at best) marginally relevant to Rhino is a bit concerning to me. Imagine the effect of every user posting like this, it would drown the board in noise 
That’s certainly not my impression (meta = being about itself i.e. the board/discussion group forms in general). But even if it explicitly was (meta = off-topic), that wouldn’t change the concern/point.
