snapshot.debian.org is an online repository where you can find current and past debian packages, organized by date, version and all source code so we can recover the previous version that worked or if we had that feature now been removed.
Currently only found versions until 2005, a few years considering that the debian project exists since 1993, but enough for the occasions where we have to "turn back" for something in particular, eg security tool provides us with the task of finding the vulnerable package, for testing in a virtualized environment to test our skills.
Source: Dragonjar.org
