Monitors servers, webpages, network devices, and anything with an SNMP interface (both queries and traps).
Has a high-availability fail-over option.
Full support for linux as well as Windows including running the monitor itself on linux.
Multiple database support including SQLserver, mySQL and Oracle.
One-click discovery of a vCenter
Alerts can be sent out via email, pager or script. (All elements of an alert can be handed to a perl script for example)
The text of email alerts can be easily customized for each alert profile.
Only requires a single port through firewalls.
Easy to create customized dashboards.
I'd like to see a tighter model for user privileges.
I'd like to have more granularity about what each group can see.
This company is extremely responsive to bug reports and feature requests. Their whole crew (sales, support, engineering) are located in Toronto, Canada.