Another commercial product with integrated PostgreSQL monitoring is Hyperic HQ: http://www.hyperic.com/products/postgresql-monitoring.
One advantage of using Hyperic's products is that they monitor a whole lot of other types of software as well. If you want to monitor your database and application server using the same technology, it may be easier to do so with Hyperic than some of the more OS or PostgreSQL focused tools mentioned here. Similarly, there are even Hyperic plug-ins aimed at monitoring connection poolers such as pgpool. Its real strength is letting you see how these various levels of your application fit together from a performance perspective. On the flip side, PostgreSQL support in Hyperic HQ is not always kept current. At the time of writing, they haven't even added support for PostgreSQL 8.4 yet.