Zenoss is a great open source system monitoring tool. I usually configure it to get data using SNMP. However, I have a few remote OpenBSD boxes that I want to keep an eye on, and I am not going to expose SNMP to the universe. Zenoss has a cool thing called Zenoss Plugins which let your collector SSH into a machine and get basic statistics. The current version of Zenoss Plugins (2.0.4) does not have support for OpenBSD.
Here's how you add it:

1. Get Python setuptools if you don't have it already. Zenoss Plugins requires setuptools in order to run.
2. Download Zenoss Plugins from SourceForge and put them on your OpenBSD machine.
3. Extract the files. You'll have a Zenoss-Plugins-2.0.4 directory.
4. Download openbsd4.py and put it in the Zenoss-Plugins-2.0.4/zenoss/plugins directory.
5. Run sudo python setup.py install
6. Test it out by running zenplugin.py cpu
This OpenBSD plugin supports the following commands:

$ zenplugin.py cpu
$ zenplugin.py mem
$ zenplugin.py disk [mount_point]
$ zenplugin.py intf [network_interface]

I have taken the liberty of assigning the copyright for this script to Zenoss, Inc. in hopes that they will include it in the next release of Zenoss Plugins.
Happy monitoring.