Some very useful options have been added to nixpkgs config:
nixpkgs.config.rewriteURL: Allows rewriting URLS downloaded by fetchurl, to configure company-internal mirrors, or work around company firewalls and similar networking restrictions.
nixpkgs.config.hashedMirrors: Allows setting content-addressed mirrors that fetchurl can fallback to if the primary URL is unavailable.
nixpkgs.config.{gitConfig, gitConfigFile}: Configures the default git config used by fetchgit. Can be used to point to local mirrors.
nixpkgs.config.npmRegistryOverrides: Configures NPM registry overrides for fetchNpmDeps.
fetchgit gained two new arguments: gitConfigFile to set a git config for an individual derivation, and rootDir to limit the resulting source to a subdirectory of the Git repository.