A person who carefully scrutinizes a magnified digital photograph in order to evaluate resolution and image quality. -Wiktionary
The above is the definition of pixel peepers by Wiktionary. It seems pretty neutral but sometimes pixel peeper can be used as a derogatory remark (see Urban Dictionary).
There are good reasons for pixel-peeping especially when you are evaluating image quality in order to make a purchasing decision on a camera or lens. Image quality can vary greatly depending on camera settings such as focal length, aperture, ISO. Different lenses may have different characteristics such as sharpness, chromatic and spherical aberration, distortion, etc.
There are many photography review sites available where the reviewers publish some objective test results and offer some subjective evaluations. The reviews may not be comprehensive enough because the reviewers may not have the resources to test every possible combination of camera/lens settings or usage scenarios. There is also a question of product quality variation. A Switzerland software engineer, Rafael Zwiegincew, realized the need for pixel peeping and launched a website called, you guessed it, pixel peeper.
The website now has a database of more than 1 million images. You can browse sample images from a particular lens or camera or search for photos based on camera settings. For example, I wanted to look for full size images taken using Nikon 35mm f/1.8G AF-S DX at f/1.8 and I found them here. In another test, I tried to find images taken using Nikon D700 with ISO from 6400 to 12800, and the results can be found here.