Reconnoiter - http://labs.omniti.com/labs/reconnoiter

Reconnoiter's goal is to better the world of monitoring by marrying fault
detection and trending together. Through ease of configuration and ongoing
maintenance, Reconnoiter encourages monitoring important technical metrics
along side critical business metrics to improve awareness and ultimately
accountability.

Reconnoiter? Who names this stuff? Honestly?
Read about the goals of Reconnoiter. Rant version.
Architecture

Web console to provide a view into the world.
Core database, holds metrics and business logic for determining faults, fault
notification and escalation procedures
Strategy daemon to collect information from Scout nodes
Scout nodes that perform checks. Scout nodes run extremely efficiently -- tens
of thousands of services monitored per scout.
A picture is worth a thousand words.

Watch the 2009 OSCON Reconnoiter presentation.

Documentation

Reconnoiter documentation
And some more ad-hoc wiki documentation
Download

Urskek Release access is available to all.

svn co https://labs.omniti.com/reconnoiter/tags/urskek reconnoiter
Subversion trunk access is available to all.

svn co https://labs.omniti.com/reconnoiter/trunk reconnoiter
Development at this point is pretty rapid. As such, decisions are made that
might cause what we call a "flag-day". A flag-day is when an upgrade through a
specific version will prevent some functionality between that version and an
older version. Commits that do this (like a change in protocol) will be marked
as "flag-day" commits and note the system components that must be lock-step
upgraded. Review flag-day commits.

Specific (significant) upgrade notes.

Installation can be a bit tricky, it might help to look at this step-by-step
guide to installing on Debian.