Your question seems to indicate you want to convert RAW files created with a Nikon camera. Several others have offered excellent answers when that is the case.
For others who may be reading this question and would like to convert RAW files created on a Canon camera, there is Canon's own Digital Photo Professional (DPP) that is included with every Canon DSLR. Updates may be downloaded free from Canon's website, but you must have a previously installed version or an original disc to install the update.
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.
Some of the features of DPP: View and sort images, display the thumbnail list at high resolution, batch rename files, and check shooting information. RAW adjustments include: brightness/darkness, shadows, highlights, Picture Style, contrast, skin tone, saturation, sharpness/unsharpen mask, white balance (color temperature, several presets, or custom), cropping (trim/angle/aspect ratio), auto or manual dust deletion, basic cloning/removal, Auto Lighting Optimizer, noise reduction (luminance and chrominance), lens aberration (distortion, CA, peripheral illumination, color blur), and Digital Lens Optimizer.
All adjustments are non destructive and contained in a "recipe" that is added to the file's metadata. Recipes can be saved and later applied to other files as well as batch applied to selected files. The recipe is applied to the image when converted and saved to JPEG or TIFF. DPP can convert and save files individually or in batches. You may also transfer a RAW image to Photoshop as a 16 bit TIFF.
There is a basic tool that can composite several images, but it is nowhere near as advanced as using layers in Photoshop. There is also an HDR tool that can be used on 1-3 files. There is tone/color control (brightness, saturation, and contrast) and Detail Enhancement (strength, smoothness, and fineness). If you use the HDR tool on RAW files, some of the adjustments made to the image in the RAW adjustment tab are carried into the HDR module (such as color temperature, dust removal/cloning, picture style, NR) while others do not appear to be (ie: saturation and contrast, which are adjusted inside the HDR tool). You may also use the HDR tool on JPEGs or TIFFs.
Update: As devices without optical drives that are capable of running their applications are becoming more common, Canon now makes available for download a version of their software suite that does not require a previous version of the disc. You may be required to enter you camera's serial number to complete the download and/or install the software.