We've been using Esko since 1996.
It's one of the best like haven stated.
We also run the full suite as well (see my sig).
Some of the features:
unlimited colors
unlimited screenings on a page
able to tag any image with a curve (or no curve), independent of any other images
full auto workflow's (hot folder, normalizing, trapping, imposing, sending to a printer) with no human intervention.
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We'll be getting the update to V4 (Software Suite 7) sometime this summer with the full live transparency functionality.
"What makes Esko's workflow automation server so sexy"
good luck,
David