I love using the built-in Equation Editor in Word 2007. Alt + = brings up the editor, then you type this:
(a+b)^n=\sum_(k=0)^n (n\atop k)a^k b^(n-k)
This will produce the following:
It doesn't get much easier than that.
There is also an entire paper (PDF) about using plaintext to type up formulas.
