Technically Ghostscript is a Postscript Interpreter rather than a RIP but if you combine Ghostscript with Ghostview and a print driver, you have a basic but functional RIP.
This is Windows only, Mac installation is rather more complicated.
Assuming that you have your printer driver installed you will need to download Ghostscript and Ghostview from http://pages.cs.wisc.edu/~ghost/
Install Ghostscript THEN Ghostview.
I would recommend sending Russell Lang at Ghostgum a few dollars for his trouble.
You need a WORKFLOW to print separations, that's just a fancy name for creating a file and printing to your printer.
To produce separations in a file you can look at this tutorial or use the Adobe Device Independent Postscript Printer which comes with your system or the Corel Postscript printer which is installed with CorelDraw.
Each method produces a file on disc with the halftoning definitions. The halftones are set in your print dialogue box.
Open the file in Ghostview.
Press print and print with your usual settings.