We just told you about the Lithium server monitoring app but what if, like most Mac users, you're more likely to monitor and control your house (or at least your lamps) than your servers? Adam Goldstein has a nice introduction to the Indigo Home Automation and Control Server over at MacDevCenter. Indigo is a Mac application for home automation using INSTEON/X10 switches, dimmers, sprinkler controllers, etc. It does this via a USB interface device which communicates with the special switches, etc. through your existing power lines. Thus, your Mac can both monitor sensors of various sorts (e.g. motion sensors) and send commands to various appliances plugged into control modules (either external ones that go between the appliance and the wall socket, or internal ones that actually replace the wall sockets). There are a variety of sensor and control modules available.
Indigo can be controlled in many ways: pre-made schedules, Applescript, Salling Clicker (Bluetooth), a Dashboard widget, the phone, or even over the internet via the built-in web server. As of now, your Mac server must remain on at all times (though a future version will be able to upload commands to the USB module and then shut off).
Indigo is not cheap at $179.95 (introductory price, $199.95 regularly, demo available) and obviously requires a considerable investment in INSTEON modules ($10 to $200 each), but if you're the geeky type like me this is just begging for a mac mini home server. The main thing missing for me is some sort of thermostat interface and perhaps more sensors.