Ultimate Debian Database (UDD) gathers a lot of data about various aspects of Debian in a single SQL database. It allows users to easily access and combine all this data.
You can have a look at the UDD Schema.
More information about UDD are collected on the Debian Wiki at wiki.debian.org/UltimateDebianDatabase.
Some example queries are provided as CGI scripts, to make it easy for everyone to run them. You can browse them or view the source code.
There are also some more complex "applications" using UDD:
If you want to know when UDD is being updated, you can check the crontab file. The timestamps table can tell you when a data source was last updated.
A full dump of the database, generated every two days, is also available. Be careful, it's quite big (400 MB gzipped, and growing).