I'd try Zenoss. I wrote a big paper comparing Nagios, OpenNMS and Zenoss ( http://www.skills-1st.co.uk/papers/jane/open_source_mgmt_options.html ) - it's nearly 2 years back now but many of the basics are the same. At that time, I plumped for Zenoss but, to be fair, my main negative on OpenNMS was that it is written in Java - it did pretty-well everything I needed.
On the Zenoss front, my main winges were the quality and reliability of the documentation and the code. Zenoss has done a HUGE amount of work on code reliability since then and the documentation is coming along. It runs Nagios plugins if you need that, and will use SNMP, WMI and ssh as protocols to talk to a device. The community is very active and, although Zenoss also have a commercial offering (Enterprise) as well as the free Core, we still seem to get very good air-time from the developers to help resolve issues and to take input from the community.
If you want further help, please ask!
Cheers, Jane