For those of you that feel like you're just starting out and don't have teh cash to spare for some "high-fallutin' screenwriting software" fear not- CeltX just upgraded their original software from a "Point 9X" to an official 1.X version.
I'm a HUGE believer in open source programs [well- that, and I'm insanely cheap] and this one is both open source & free!
It has storyboard integration, production statistical referencing- it'll even help you plot out the shot score and plan the calendar for your shooting and FX schedules!
Is it Movie Magic Screenwriter 6? No- perhaps not...
But gosh darnit- it is incredibly good- especially for free!