Nagios is a well-known monitoring tool. We won't make any claims that it is the best or most suitable tool for watching a highly available PostgreSQL installation. However, the community is large, the functionality is extensive and established, and interoperability with other tools and libraries is high. It was also one of the first to attain prominence and, as such, is a good way to learn how most monitoring systems function.
As an unfortunate consequence, the amount of installation prerequisites is rather lengthy. To get Nagios working properly, we need an HTTP server, Perl, and a mail daemon. Some plugins require PHP, while others need MySQL, SNMP (short for Simple Network Management Protocol) or any number of esoteric utilities and acronyms. There might be DBAs who also have strong skills as webmasters, but we can't depend on that...