Take and update mirrors of any Debian/Ubuntu remote repository.
Manage your own repositories of packages, merge them with offical and publish the result.
At any point in time, take snapshot of the mirror to fix current set of packages.
Any snapshot could be published back as repository (only HTTP server is required).
Merge two or more snapshots into one snapshot: e.g. merge wheezy and wheezy-updates.
New package version available in backports? Pull only single package with dependencies from backports into snapshot of stable repository.
Don't donwload packages that are not required, controlling dependencies between them.
Publish your repositories directly to Amazon S3 as public or private repositories.
Search for specific packages, their inclusion into snapshots, etc.
Mirror official Debian repositories (as snapshots) without resigning by your own key.