While we have an product that is installed on a Windows host, and monitors Windows in general, we always taken pride in knowing that Windows is just part of the datacenter (something our competitors mostly ignore). After an agonizing week of testing we finally completed our support for monitoring *nix systems.
Linux (Red Hat, Fedora, CentOS)
BSD (Free BSD, OpenBSD)
AIX (4.2 and forward)
We included support for CPU, memory, swap, NIC utilization, disk utilization as standard for these OS's , and its all agentless. I think we are pretty unique to do that.
Did we miss anyone ? Let us know.