How do you add PostgreSQL Driver as a dependency in Maven?

PostgreSQL drivers jars are included in Central Repository of Maven:

• List of included versions of PostgreSQL drivers.

For PostgreSQL up to 9.1, use:

<dependency>
    <groupId>postgresql</groupId>
    <artifactId>postgresql</artifactId>
    <version>VERSION</version>
</dependency>

or for 9.2+

<dependency>
    <groupId>org.postgresql</groupId>
    <artifactId>postgresql</artifactId>
    <version>VERSION</version>
</dependency>

(Thanks to @Caspar for the correction)