A free plugin available for R15 and up at the downloads page
The matrix draw helper plugin is a workflow tool that allows you to visualise more in the viewport when working with mograph, whilst keeping viewport speeds nice and friendly.
You can't render anything with this plugin (unless you're using a hardware mode etc) but that's why it works so fast. As there's no geometry or object overhead, the system is able to draw lots of content into the viewport so you can get your timing right for animating whilst using some of the different modes to help you see things like animated effects, clone direction and even some basic lighting.
I'd love to see what uses you can find for this. I already use it every day, hopefully it'll be useful for some of you too.