ESXi monitor

VMware vCenter Server

you've also got

Veeam Monitor and Reporter - Monitoring and Alarming, Capacity Planning.
vKernel - Optimization and Trending (if you need it!)
Quest vFoglight

spriceworks: http://www.spiceworks.com
Nagios: http://www.nagios.org/
PRTG: http://www.paessler.com/prtg
Opmanager: http://www.manageengine.com/datacenter-and-server-management.html

http://exchange.nagios.org/
http://www.linuxjournal.com/article/6767
http://www.nagioswiki.com/wiki/index.php/Main_Page
http://m.linuxjournal.com/files/linuxjournal.com/ufiles/LJ191_UsingSMSforNagios.pdf
http://www.groundworkopensource.com GroundWork Monitor Enterprise
http://www.hyperic.com Hyperic HQ
http://www.icinga.org Icinga
http://oss.oetiker.ch/mrtg/  mrtg
http://www.nagios.com Nagios
http://www.n-able.com N-central
http://www.op5.com op5 Monitor
http://www.open-itcockpit.com openITCockpit
http://www.opennms.org OpenNMS
http://www.hp.com/de/bto HP OpenView
http://www.bmc.com BMC PATROL ProactiveNet Performance Management
http://www.shinken-monitoring.org Shinken
http://www.ibm.com/software/de/tivoli IBM Tivoli
http://www.ca.com/de CA Unicenter
http://www.whatsupgold.com WhatsUp Gold
http://www.zabbix.com Zabbix
http://www.zenoss.com Zenoss Enterprise

Splunk http://www.splunk.com/ 
Cisco CS-MARS http://www.cisco.com/en/US/products/ps6241/index.html
Sawmill:  http://www.sawmill.net
Paglo:  http://paglo.com
Q1 Labs (offers free VM version):  http://www.q1labs.com/qradar-slim-fe
Manage Engine with OpManager:  http://www.manageengine.com

Thanks.

Good stuff @hanccocka! :)