You can use the web npm web site as it lists the dependents for any given package.

Like coffee-script for example:

http://npmjs.org/browse/depended/coffee-script

Or, you could get JSON results for the same query (using curl) (just substitute the name of the package with "sql" in the example:

> curl -g 'http://registry.npmjs.org/-/_view/dependedUpon?group_level=2&startkey=["sql"]&endkey=["sql","ZZZZZZZZZZZZZ"]&skip=0&limit=1000'

Results:

{"rows":[
{"key":["sql","anydb-sql"],"value":1},
{"key":["sql","dbal"],"value":1},
{"key":["sql","ectypes-downstairs"],"value":1},
{"key":["sql","fixr"],"value":1},
{"key":["sql","fixr-compiled"],"value":1},
{"key":["sql","forerunner-postgres-store"],"value":1}
{"key":["sql","pg-dal"],"value":1},
{"key":["sql","relational"],"value":1},
{"key":["sql","sequelize"],"value":1},
{"key":["sql","sql-generate"],"value":1},
{"key":["sql","sqlbox"],"value":1},
{"key":["sql","triplie"],"value":1},
{"key":["sql","voltron-postgres-adapter"],"value":1},
{"key":["sql","worm"],"value":1}
]}

The second value in the key array is the name of the package dependent on the sql package.

This of course only will find other published packages, and doesn’t help find applications, etc. that are using a specific package.