Jon Udell - Del.icio.us is a database:

"Although it's intuitively obvious to me, I suspect that most people don't yet appreciate how easily, and powerfully, tagging systems can work as databases for personal (yet shareable) information management.

Del.icio.us isn't simply backed by a database, it can function as a database to which you add (a lot of) queryable columns.

[...] It strikes me that there's a sweet spot somewhere between this shoestring approach and the likes of Dabble DB, an application that offers powerful web-based data management. Consider how dBase and later Access were overkill for most people's recipe lists and address books, and how 1-2-3 and Excel wound up meeting the need instead. Tag systems might turn out to be the spreadsheets of modern information management."