http://www.mathtype.com/en/products/mathtype/

“The best thing for writing equations since chalk”, erm not quite. Pen and paper still wins in my book. However this program is very good at producing lovely looking maths equations as images, and then you can paste them easily to Word etc.

It is only a free as a trial though, but I think turning back your computer clock stops it expiring.

See also: http://www.physlink.com/Education/software_mathtype4.cfm
http://evolve.lse.ac.uk/software/EquationEditor/