Search for software in Nix channels:

https://lazamar.co.uk/nix-versions/

Use case: Search for old software in older Nix channels, e.g. earlier verison of binutils (bintools):

https://lazamar.co.uk/nix-versions/?channel=nixos-18.09&pack...

Instructions for how to install/use this earlier version of bintools on NixOS:

https://lazamar.co.uk/nix-versions/?package=binutils&version...

For example, use an old binutils (2.23.1) in an ephemeral shell:

nix-shell -p binutils -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/23befca742f67494f44...

(In my opinion, a perfect complement for regular NixOS package search: https://search.nixos.org/packages)