Database adapters for Node are beginning to show up - there's an async PostgreSQL one and I think there's one using libdrizzle (which should work for MySQL as well). Since Node speaks HTTP fluently it's great for talking to things like CouchDB, and there's also DBSlayer http://code.nytimes.com/projects/dbslayer which provides an HTTP API for running MySQL SQL queries.

Node + Redis is an awesome combination.

Don't forget APIs - if your application simply hits a bunch of HTTP APIs from other providers (mashup style) Node is a great fit.