Hi,
better than equation service imho is LaTeXit! Just drag n drop your equation in a vector pdf format, and create your custom stencil. The linkback allows you to change your equation afterwards! (on the next screenshot I'm changing the color in LaTeXit, and the change also affects the equation in Omnigraffle)
(sure, you'll need to know some LaTeX... but have a look at this screenshot and tell me it's not an obvious notation!)
\int --> integral
\frac{1}{2} --> fraction
\alpha, \beta, \omega, \lambda, \dots
\frac{\partial^2 f}{\partial x^2}
|x|=\left\{
\begin{array}{l} --> array, fields aligned on the left
+x\textrm{, if }x\geq 0\\
-x\textrm{, if }x\leq 0
\end{array}
\right.
