Binary JAR file downloads of the JDBC driver are available here and the current version with Maven Repository. Because Java is platform neutral, it is a simple process of just downloading the appropriate JAR file and dropping it into your classpath. Source versions are also available here for recent driver versions.
This is the current version of the driver. Unless you have unusual requirements (running old applications or JVMs), this is the driver you should be using. It supports PostgreSQL 8.2 or newer and requires Java 6 or newer. It contains support for SSL and the javax.sql package.
If you are using Java 8 or newer then you should use the JDBC 4.2 version.
If you are using Java 7 then you should use the JDBC 4.1 version.
If you are using Java 6 then you should use the JDBC 4.0 version.
Many other versions of the JDBC driver are available. This includes development versions, compatibility with older JDKs, and previous versions of the driver.