As real time as you can get

The people from Orbitz (the popular travel site) open sourced their real time monitoring application called Graphite.

It uses an specialized database to store the time series data. This allows to quickly retrieve the monitoring information. Other approaches like storing directly to a SQL database are slower because of the engine´s overhead.

Graphite has a very slick and elegant UI to browse the monitored metrics and a CLI for faster administration.

I´m currently using Zabbix at the office, but it needs lots of maintainance from the operations people and has some glitches when it comes to gather the metrics. It also interpolates data when it's inexistent. A critical flaw for a monitoring application. We are planning to replace it with another tool ASAP.