Netfabb Free is super fast, free, and easy to use.
It has sliders for X, Y, & Z axis so you can just move slider to where you want to make the cut, click apply, and then it creates 2 parts. You export both parts and you're done. No mucking about with hole filling, selection tools, watertight, bla bla bla, just cut, export, print.
Handy .pdf right here: