A computer - and not just any computer, my computer.  The workflow is built into the various applications that I use from CAD programs to billing.  A tool belt might be a physical metaphor for how it works on the office side of the business.  I'm constantly pulling another tool up on the screen to process information, orders, manage communication with clients, manage supply chain issues, and of course design, build, and bill for clients.

It might not be obvious, but the defined workflow helps with responsibility through out the project and my clients don't have to manage the work.  That's what I do.  They are going to their kids soccer games or taking a vacation instead of spending their weekend at big box home improvement store trying to figure out what to buy and how it's going to fit in the back of their Lexus.