• Intro to the Nix language at Nixcloud
• Nix Pills (getting started guide)
• Package search
• NixOS options search
• home-manager options search
• nix-darwin options search
• Official wiki
• Official Nix documentation at nix.dev
• Function search at Noogle
• Old package version searches
  • https://www.nixhub.io
  • https://lazamar.co.uk/nix-versions

Diffing system builds

Build with nixos-rebuild build and then run the following to see the package versions that will change when you switch:

nix store diff-closures /run/current-system ./result

Other useful utilities

nix-tree will let you see a tree of required store paths, and can be used on a system build:

nix-tree ./result