To be honest I was unsure exactly what ACR was/is. Is it a free download plugin for PS3
I would not exactly call it "free" any more than I would call maintenance updates to CS3 as being "free".
It is part of the CS3 suite, but it can run stand alone if you are just running it as a RAW converter. In that context, it functions as DPP does. After RAW conversion, DPP files are often imported into Photoshop for final editing/filtering/sharpening/printing which is somewhat analogous to the way that ACR and Photoshop work together ... although with smart objects, the integration between ACR and Photoshop becomes much tighter than that.