I can't believe I've been converted to the point of saying this, but if you're just getting started you might want to have a go at using VPA instead of Winplan. I started w/ Winplan, and certain games pretty well forced the use of something else, so I gave it a shot... and now all the little extras it helps with are invaluable to me. It takes a bit to get used too, but it's worth it, really.
That speech aside, I'd say something like the Randmax/Randgen combo, or Tanascius (sp?), to help out with economics. Not required, and some prefer not to let the computer put its hands on that area, but it's quite nice for general economic buildup, and you can tweak things on your own on top of it.
SendTrnPro is handy too -- especially if you're in several games. It can be integrated with Winplan, but even if you use another client, it still makes turn submission much quicker.
Last but not least on my list is VPMagic -- it's a good central point for starting games, running utilities, etc etc.
Okay, an addition -- not so much a utility, but quite possible the most important VGAP helper out there: Donovan's website
. Knowing the order of things during host, and the specifics of combat, etc, can make a HUGE difference.