That depends on what you're trying to monitor. The monitoring ecosystem is quite fragmented (see MonitoringSucks), so there are huge variety of tools for different needs. Examples:
Log aggregation/search/alerting: logstash, loggly, papertrail, Sumo Logic.
Availability monitoring: Keynote, Pingdom, Uptime Robot, Monitority.
Application monitoring: New Relic, App Dynamics, ruxit, Datadog
Hardware monitoring: Nagios, Icinga, Munin, Ganglia, Cacti, sensu