We've talked before about how to use AutoVoice, the excellent Tasker plugin, to create a custom voice command for anything on Android. Now, the plugin allows you to tie those commands directly into Google Now, so you can trigger anything you want with a simple "OK, Google".
The bulk of the work hasn't changed much from our previous guide. If there's a task you can accomplish with Tasker, you can trigger it via a custom voice command. The difference is that now, AutoVoice can use a listening service to draft on Google Now's voice commands. That means you can launch your voice commands from anywhere you can access Google Now, which is an ever-growing category of devices and situations.