SQLite is the hottest new thing on the block.
For any use case, SQLite is one of the best ways to persist structured data to local disk.
Its a little scary how much it can actually handle, and how lazy and sub optimal it is to reach for a "real" sql server before you need it.
Having the database as a simple file right next to yoyr application is incredibly convenient not to mention how brain dead things like backups are to grok.