I would like my WiFi to automatically disconnect when I plug an Ethernet cable into my Mac. How would I go about this?
If you have somehow modified the default service ordering, you can set it again in the network tab using the gear.
This makes the Wi-Fi inactive for internet routing whenever you connect the cable. What you ask is scriptable if you know some programming, but there isn't a simple trigger to actually turn the Wi-Fi radio off at the user level of the OS.
This script on MacOSXHints worked for me up until installing Mavericks (10.9.0) on a new Late 2013 MBP retina 15" - but it did work quite well on my previous two non-retina MBPs before that.
Note that you may need to edit the script to use your correct interface numbers (ie en1).