Software for system flow diagrams?

I am in the mock up stage on my brewery upgrade, and I need to do some work equpitment placing and visualizing the flow for different steps. what is everyone using to make all these nice diagrams? Don't need nuthin too fancy, just something that will produce a coherent result. Much obliged!

MS Visio, or Google sketchup

Go Visio if you are familiar with Office-type products. They also have a free trial

+1 on sketchup

A lot of people have already created some objects that may be of use to you...

I am modeling my system extensively on sketchup, and actually planning to put together a home brew component library of sorts of all the parts I have spent time modeling to save people from further reinventing the wheel. Having said that, I am looking for a more simplified diagram so I can figure out where I want valve, components etc, before I model them. Is there a basic and/or 2d mode in sketchup, preferably with a grid?