Stuff like SpiceWorks promises so much, but we have had mixed experience.
At my workplace we use Active Directory and a few scripts we rolled ourselves and that works fine. We're a small company, so we don't need a huge IT management suite.
I've heard good things about Zenoss (http://www.zenoss.com/), but no first-hand experience with them.
In a larger company, I can see IT management software being useful. If you have 30+ users, you really should have it. If you have less than 10, it's probably not worth the effort.
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.
At a previous employer, we used FiveRuns and it was great. Not too expensive and it was nice knowing that I'd not have to answer "How did these systems go down and we weren't notified" with "Because the monitoring system was one of the ones that went down". Great service team, too.
For network inventory, I recommend using Lansweeper. Lansweeper is a powerful freeware solution without any embedded ads to make a complete software, hardware, computer inventory of your windows network.
