I've tried spiceworks, zabbix, and zenoss and I would say zenoss is my favorite. Spiceworks seemed to get pretty slow once I started adding in all of my machines assets a few hundred tickets. Zabbix was painful just getting up and running for me, and it seemed like a lot of work to use for a small environment. The Zenoss virtual appliance was incredibly easy to setup (on vmware server 2.0, ESX didn't like it) and get configured.
If you want to use snmp with zenoss and get detailed information you need to install snmp informant which was less than ideal for me, but there is a zenpack that allows you to get a cornucopia of data using WMI that's working well for me.