The camera manufacturer can sometimes offer an excellent RAW->JPG convertor. One reason to use the manufacturer's software is that no one else knows better how to interpret the RAW information. All the light and lens-specific data especially can be quite tricky to fully interpret and post-process by other than the manufacturer of the camera.

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In the Nikon world, there's ViewNX, which ships for free with the DSLRs and is also downloadable for free here. It's excellent for first-pass editing of photos, including Exposure, White Balance, Sharpness, Contrast, Brightness, Highlight and Shadow Protection (very impressive), Color Booster, D-Lighting HS, and Axial Color Aberration. You can also do all your Metadata edits here.

Of course, it's not as full-featured as their expensive, and terribly slow pay version: CaptureNX.

UPDATE: Nikon's Capture NX-D is now free

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Canon's own Digital Photo Professional (DPP) is included with every Canon DSLR. It can be downloaded for free from Canon's website, but you must have a valid camera serial number to download it. Apart from the obvious lack of no additional expense, the primary advantage to using DPP is that the same proprietary algorithms used to encode .crw and .cr2 files are used to decode them. It has a fairly full list of features of non destructive adjustments that can be made on a global level including a basic HDR tool. RAW files may be exported as 16 bit TIFFs to other image editors for further adjustment when desired. It features the Digital Lens Optimizer (DLO) which corrects for several lens aberrations (spherical aberration, curvature of field, astigmatism, comatic aberration, sagittal halo, chromatic aberration of magnification, axial chromatic aberration).

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For Sony cameras it would be the Image Data converter software. It used to be two separate programs called Image data lightbox and Image data converter SR, but they combined those into one package in 2012. No requirements for download, as there is for Canon and Olympus. It processes RAW files, but offers next to nothing for images already in JPEG format. Also RAW-features are limited - for example you can't crop and resize at the same go. You can convert one RAW-image, save the recipe and then apply it in a batch process to other images without a need to open each RAW-file separately.

Link to Sony eSupport software pages

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Olympus offers Image Viewer 3 for Olympus camera owners. The download will not begin without a camera serial-number filled in a field on the download page. Image Viewer 3 is a nice upgrade from the old Olympus Master 2 and the not-so-old Image Viewer 2. Selection of possible operations is good for RAW and also for images already in JPEG format. When saving to JPEG you can also include IPTC info in the file.

Link to Olympus software download

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