NixOS search
From https://search.nixos.org you can search nixpkgs and NixOS options.
Search packages
The package search searches among nixpkgs packages in either the stable release or unstable. For the returned packages, you can view the version, and follow links the the nixpkgs source and to the program's home page.
See also Searching packages
Search options
The options search allows searching of NixOS module options. For each option, you are presented with a description, example and the default setting. There is also a link to the source code of the module that defines the option.