Apt-offline is an offline apt package manager.
apt-offline can fully update and upgrade an apt based distribution without connecting to the network, all of it transparent to apt.
apt-offline can be used to generate a signature on a machine (with no network). this signature contains all download information required for the apt database system. this signature file can be used on another machine connected to the internet (which need not be a debian box and can even be running windows) to download the updates. the downloaded data will contain all updates in a format understood by apt and this data can be used by apt-offline to update the non-networked machine.
apt-offline can also fetch bug reports and make them available offline.