I am a pretty hardcore ZenOSS user.. We use it to monitor over 1000 devices in different fashions - using a combination of SNMP (Linux), WMI (windows) and SSH (Unix/Aix). While there is a slight learning curve to get everything working the way you want - it is, in my opinion, the most powerful open source NMS. It is a very active project with excellent community (and even commercial) support.
Yeah, the WMI capability is a big plus for me... I am downloading the pdf's to print for some weekend reading.