If camera manufactures would provide the same algorithm as a software package that the camera uses when processing RAW to JPG in camera.

That way, one could shoot raw, download the files to their computer, convert to JPG (just like the camera would), and then tweak.

Maybe this already exsists, if so I don't know about it.

I think Canon Digital Photo Professional does what you're implying (if I'm understanding the question correctly). I think it reads the RAW file and applies the picture style you choose in camera and such, but I'm not totally sure of that. If you use Nikon, then I have no idea lol

Pretty sure that DPP does just that. No clue about Nikon though.

From what I understand and my experience, DPP has it's own conversion engine which is quite different from in-camera, which is why it was able to offer Picture Styles for camera's which didn't support them. It has different controls and graduations (1 through 10 vs -3 to +3, for example)

It's ZoomBrowser and it's RIT (Raw Image Task) that mimics very closely if not perfectly the in-camera processing. If you look at the raw processing control panel, it will change depending on the source of the image. My XT CR2's didn't have picture style adjustments whereas my 30D raw files do. I only wish it could convert as fast as the DSLR.

if somebody wants to test, they can shoot a RAW+Large JPG and do default conversions in DPP and ZB. Load each with the in-camera jpgs as separate layers in PS and use the "difference" blending mode to check for similarity. I don't have any jpgs to test with myself.